Thread: Re: [HACKERS] Hint Bits and Write I/O
On Tue, 2008-05-27 at 19:32 -0400, Tom Lane wrote: > Simon Riggs <simon@2ndquadrant.com> writes: > > My proposal is to have this as a two-stage process. When we set the hint > > on a tuple in a clean buffer we mark it BM_DIRTY_HINTONLY, if not > > already dirty. If we set a hint on a buffer that is BM_DIRTY_HINTONLY > > then we mark it BM_DIRTY. > > I wonder if it is worth actually counting the number of newly set hint > bits, rather than just having a counter that saturates at two. We could > steal a byte from usage_count without making the buffer headers bigger. > > > If the bgwriter has time, it will write out BM_DIRTY_HINTONLY buffers, > > though on a consistently busy server this should not occur. > > What do you mean by "if it has time"? How would it know that? > > > This won't change the behaviour of first-read-after-copy. To improve > > that behaviour, I suggest that we only move from BM_DIRTY_HINTONLY to > > BM_DIRTY when we are setting the hint for a new xid. If we are just > > setting the same xid over-and-over again then we should avoid setting > > the page dirty. So when data has been loaded via COPY, we will just > > check the status of the xid once, then scan the whole page using the > > single-item transaction cache. > > This doesn't make any sense to me. What is a "new xid"? And what is > "setting the same xid over and over"? If a page is full of occurrences > of the same xid, that doesn't really mean that it's less useful to > correctly hint each occurrence. > > > The whole proposal seems a bit overly complicated. What we talked about > at PGCon was simply not setting the dirtybit when setting a hint bit. > There's a certain amount of self-optimization there: if a page > continually receives hint bit updates, that also means it is getting > pinned and hence its usage_count stays high, thus it will tend to stay > in shared buffers until something happens to make it really dirty. > (Although that argument might not hold water for a bulk seqscan: you'll > have hinted all the tuples and then very possibly throw the page away > immediately. So counting the hints and eventually deciding we did > enough to justify dirtying the page might be worth doing.) WIP patch, for discussion and performance evaluation. This patch changes the way that buffer hints are managed. We can split the patch conceptually into two parts: how we set buffer hints and what happens to buffers for which hints have been set. With this patch, when we set a hint on a buffer we don't dirty the buffer, though we keep track of hint_count for each buffer. Note that there is no additional block state, no BM_SET_HINT_BITS etc.. When running a VACUUM command we always dirty the block when setting hint bits, for a number of reasons: * VACUUM FULL expects all hint bits to be set prior to moving tuples * Setting all hint bits allows us to truncate the clog * it forces the VACUUM to write out its own dirty buffers, which is OK, since it is a background process. Other commands call HeapTupleSatisfiesVacuum(), yet these tasks can be more flexible with hint bit setting. These include ANALYZE, CREATE INDEX, CLUSTER, HOT pruning and index scan marking deleted tuples (with changes in all index AMs). This means we have to differentiate between VACUUM and other callers of HeapTupleSatisfiesVacuum(). So the patch changes the APIs of HeapTupleSatisfiesVacuum(), SetBufferCommitInfoNeedsSave() and SetHintBits() with changes to 13 AM and command files. There are many changes in tqual.c, which seems the right way because SetHintBits() is inlined. These make the patch fairly large, though most of it is simple changes. Hints are set on buffers in various ways, most of these ways relate to tuples, though there are other hints which touch the block itself. This patch takes the simply assumes that all hints are equal. We might later choose, for example, that index LP_DEAD hints are worth more than heap visibility hints. We might also argue that CLUSTER hints should *never* set the page dirty, since those will be wasted writes if the CLUSTER's transaction commits. We don't yet do anything like that in buffer usage_count, so no need to get that complex yet in this case. So what happens to buffers that have hint_count > 0? During the bgwriter's normal LRU scan we write out non-dirty buffers when the number of buffer hints is greater than or equal to a new GUC parameter called bgwriter_hints_force_write. The default and minimum value for this parameter is 1, so very similar to existing behaviour. Expected settings would be 2-5, possibly as high as 20, though those are just educated guesses. So the maximum is set arbitrarily as 100. During a checkpoint we write out only fully dirty buffers, which are the only ones required for correctness. The bgwriter does interrupt itself to re-commence the LRU scan, so the bgwriter may still write hinted blocks during the time a checkpoint is in progress. Doing it this way ensures we don't confuse the two separate goals of correctness and performance, which currently contributes to the peak write workload during checkpoint. When buffer replacement takes place in a backend, we only write out a victim buffer if it is dirty. We *never* write out non-dirty victim buffers, whatever their hint count. This speeds up the path for backends if/when the bgwriter is ineffective and is a change from the current behaviour. It also avoids adding to the most frequent code path when blocks aren't hinted at all. Temp buffers are never dirtied by hint bit setting. Most temp tables are written in a single command, so that re-accessing clog for temp tuples is seldom costly. This also changes current behaviour. We might imagine a slightly smarter bgwriter algorithm or an auto-tuning mechanism, but this patch is intended to allow the basic principles to be tested and proven before we worry about such niceties. I've got a few ideas, just as you probably have by now while reading this. We set hint_count = 0 only when buffer allocated or assigned from freelist, minimising additional management overhead. hint_count is ignored once the buffer has been dirtied. All of this should have the effect that a table scan on a newly loaded table will avoid re-writing the table. The scan sets the hints but doesn't dirty the block, so when it loops around to reuse buffers it will not need to write them out. The bgwriter may clean some of the blocks, if it has time, but that's it. There is one minor strangeness in the patch, which is the change of initdb's command order when "vacuuming database template1". With the previous ordering of ANALYZE; VACUUM FULL; VACUUM; the flexible hint bit setting of the ANALYZE on a freshly bootstrapped database caused a *consistent* error during the VACUUM FULL which follows it. That took, (cough, splutter), a little while to resolve. I've added that as a test to the vacuum regression tests and not found another error (yet?). An interesting mystery though. :-) Some instrumentation can be added - I'll add what people think is useful for attempting to prove this is effective. I am expecting the patch to reduce I/O on both large and small databases, though possibly at the expense of scalability. Let's see. Comments and performance test results, please. I expect to continue working on the patch for a while yet. backend/access/gist/gistget.c | 4 backend/access/hash/hash.c | 6 ! backend/access/heap/heapam.c | 6 ! backend/access/heap/pruneheap.c | 10 !! backend/access/index/indexam.c | 7 ! backend/access/nbtree/nbtinsert.c | 6 ! backend/access/nbtree/nbtree.c | 2 backend/access/nbtree/nbtutils.c | 6 ! backend/catalog/index.c | 3 backend/commands/analyze.c | 9 ! backend/commands/cluster.c | 6 ! backend/commands/vacuum.c | 10 !! backend/commands/vacuumlazy.c | 6 ! backend/storage/buffer/buf_init.c | 1 backend/storage/buffer/bufmgr.c | 69 +!!!!!!!!!!!!! backend/storage/buffer/localbuf.c | 1 backend/utils/misc/guc.c | 9 + backend/utils/misc/postgresql.conf.sample | 2 backend/utils/time/tqual.c | 140 !!!!!!!!!!!!.... bin/initdb/initdb.c | 2 include/storage/buf_internals.h | 3 include/storage/bufmgr.h | 3 include/utils/tqual.h | 4 test/regress/expected/vacuum.out | 2 test/regress/sql/vacuum.sql | 2 25 files changed, 39 insertions(+), 280 modifications(!) -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
Attachment
Simon Riggs wrote: > When running a VACUUM command we always dirty the block when setting > hint bits, for a number of reasons: > * VACUUM FULL expects all hint bits to be set prior to moving tuples > * Setting all hint bits allows us to truncate the clog > * it forces the VACUUM to write out its own dirty buffers, which is OK, > since it is a background process. > > Other commands call HeapTupleSatisfiesVacuum(), yet these tasks can be > more flexible with hint bit setting. These include ANALYZE, CREATE > INDEX, CLUSTER, HOT pruning and index scan marking deleted tuples (with > changes in all index AMs). This means we have to differentiate between > VACUUM and other callers of HeapTupleSatisfiesVacuum(). > > So the patch changes the APIs of HeapTupleSatisfiesVacuum(), > SetBufferCommitInfoNeedsSave() and SetHintBits() with changes to 13 AM > and command files. There are many changes in tqual.c, which seems the > right way because SetHintBits() is inlined. These make the patch fairly > large, though most of it is simple changes. If only VACUUM is going to set "flexible" to off, maybe it's better to leave the APIs as they are and have a global that's set by VACUUM only (and reset in a PG_CATCH block). -- Alvaro Herrera http://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support
On Wed, 2008-06-18 at 14:53 +0100, Simon Riggs wrote: > There is one minor strangeness in the patch, which is the change of > initdb's command order when "vacuuming database template1". With the > previous ordering of ANALYZE; VACUUM FULL; VACUUM; the flexible hint bit > setting of the ANALYZE on a freshly bootstrapped database caused a > *consistent* error during the VACUUM FULL which follows it. That took, > (cough, splutter), a little while to resolve. I've added that as a test > to the vacuum regression tests and not found another error (yet?). An > interesting mystery though. :-) Ah! Now I understand. The ANALYZE was setting hint bits, yet not dirtying the buffer. When the VACUUM reads the buffer it sees the hint bits set, so doesn't set the buffer dirty. Yet if the buffer is replaced the hints are lost, yet the VACUUM now relies upon their presence - wham! So, for this to work VACUUM correctly must dirty any buffer it touches that has hint_count > 0, even if no hints were set by the VACUUM. VACUUM will then act the same, no matter whether another session has recently touched the buffer. Conceivably, this might mean that VACUUM dirties *more* buffers than it did before, but at least it will write them also. New version on its way. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
On Wed, 2008-06-18 at 23:41 +0100, Simon Riggs wrote: > New version on its way. New version complete, but I'm doing some more performance profiling before submitting next version. If anybody is waiting, just shout and I'll post the current version. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
"Alvaro Herrera" <alvherre@commandprompt.com> writes: > If only VACUUM is going to set "flexible" to off, maybe it's better to > leave the APIs as they are and have a global that's set by VACUUM only > (and reset in a PG_CATCH block). Ugh. Perhaps it would be simpler to have a wrapper function HTSV() macro which passes flexible=true to HTSV_internal(). Then vacuum can call HTSV_internal(). I'm not sure what the performance tradeoff is between having an extra argument to HTSV and having HTSV check a global which messes with optimizations. -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Ask me about EnterpriseDB's RemoteDBA services!
"Simon Riggs" <simon@2ndquadrant.com> writes: > The default and minimum value for this parameter is 1, so very similar to > existing behaviour. Expected settings would be 2-5, possibly as high as 20, > though those are just educated guesses. So the maximum is set arbitrarily as > 100. Not a fan of arbitrary constants. ISTM this should just have a maximum of MaxHeapTuplesPerPage. I'm not really happy with having this parameter at all. It's not something a DBA can understand or have any hope of setting intelligently. I assume this is a temporary measure until we have a better understanding of what real-world factors affect the right values for this knob? > Temp buffers are never dirtied by hint bit setting. Most temp tables are > written in a single command, so that re-accessing clog for temp tuples > is seldom costly. This also changes current behaviour. I'm not sure I agree with this logic and it doesn't seem like temporary tables are an important enough case to start coming up with special cases which may help or may hurt. Most people use temporary tables the way you describe but I'm sure there's someone out there using temporary tables in a radically different fashion. I'm also a bit concerned that *how many hint bits* isn't enough information to determine how important it is to write out the page. What about how old the oldest transaction is which was hinted? Or how many *unhinted* xmin/xmax values were found? If HTSV can hint xmin for a tuple but finds xmax still in progress perhaps that's a good sign it's not worth dirtying the page? -- Gregory Stark EnterpriseDB http://www.enterprisedb.com Get trained by Bruce Momjian - ask me about EnterpriseDB's PostgreSQL training!
On Fri, 2008-06-27 at 15:25 +0100, Gregory Stark wrote: > "Alvaro Herrera" <alvherre@commandprompt.com> writes: > > > If only VACUUM is going to set "flexible" to off, maybe it's better to > > leave the APIs as they are and have a global that's set by VACUUM only > > (and reset in a PG_CATCH block). > > Ugh. Perhaps it would be simpler to have a wrapper function HTSV() macro which > passes flexible=true to HTSV_internal(). Then vacuum can call HTSV_internal(). > > I'm not sure what the performance tradeoff is between having an extra argument > to HTSV and having HTSV check a global which messes with optimizations. Doing this doesn't actually reduce the size of the patch much, as it turns out, so I suggest we don't do this. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
On Fri, 2008-06-27 at 15:36 +0100, Gregory Stark wrote: > "Simon Riggs" <simon@2ndquadrant.com> writes: > > > The default and minimum value for this parameter is 1, so very similar to > > existing behaviour. Expected settings would be 2-5, possibly as high as 20, > > though those are just educated guesses. So the maximum is set arbitrarily as > > 100. > > Not a fan of arbitrary constants. ISTM this should just have a maximum of > MaxHeapTuplesPerPage. > > I'm not really happy with having this parameter at all. It's not something a > DBA can understand or have any hope of setting intelligently. I assume this is > a temporary measure until we have a better understanding of what real-world > factors affect the right values for this knob? Yes, its a guess at what sort of control we'll need. > > Temp buffers are never dirtied by hint bit setting. Most temp tables are > > written in a single command, so that re-accessing clog for temp tuples > > is seldom costly. This also changes current behaviour. > > I'm not sure I agree with this logic and it doesn't seem like temporary tables > are an important enough case to start coming up with special cases which may > help or may hurt. Most people use temporary tables the way you describe but > I'm sure there's someone out there using temporary tables in a radically > different fashion. Thanks for your comments. The patch splits into two parts: * the machinery to *not* dirty a page when we set hints * behaviour modifications now that we can tell the difference between dirty and hinted pages Nobody has yet come up with any comments about the first half, which is good. The second part is clearly where much debate will occur. I'm going to literally split the patch into two, so we can get the machinery into CVS and then fiddle and argue over the second part over next few months. > I'm also a bit concerned that *how many hint bits* isn't enough information to > determine how important it is to write out the page. What about how old the > oldest transaction is which was hinted? Or how many *unhinted* xmin/xmax > values were found? If HTSV can hint xmin for a tuple but finds xmax still in > progress perhaps that's a good sign it's not worth dirtying the page? Sounds interesting. We can track anything and everything really, but we do need to come to a firm dirty/not decision at some point. If you can develop those ideas a bit more by Monday, I'll try to put them in the patch. (I'm away until then now). -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
Gregory Stark wrote: > I'm also a bit concerned that *how many hint bits* isn't enough information to > determine how important it is to write out the page. Agreed, that doesn't seem like a very good metric to me either. > Or how many *unhinted* xmin/xmax > values were found? If HTSV can hint xmin for a tuple but finds xmax still in > progress perhaps that's a good sign it's not worth dirtying the page? I like that thought. Overall, I feel that we should never dirty when setting a hint bit, just set the separate buffer flag to indicate that hint bits have been set. The decision to dirty and write out, or not, should be delayed until we're about to write/replace the buffer. That is, in bgwriter. How about this strategy: 1. First of all, before writing a dirty buffer, scan all tuples on the page and set all hint bits that can be set. This will hopefully save us from having to dirty the page again in the future, when another tuple on the page is accessed. This has been proposed before, and IIRC Tom has argued that it's a modularity violation for bgwriter to access the contents of pages like that, but I'm sure we can find a way to do it safely. 2. When bgwriter encounters a page that's marked as "hint bits dirty", write it only if *all* hint bits on the page has been, or can be, set. Dirtying a page before that point doesn't seem worthwhile, as the next access to the tuple that doesn't have all the hint bits set will have to dirty the page again. Actually, I'd like to see some benchmarks on an even simpler strategy: just never dirty a page just because a hint bit has been set. It might work surprisingly well in practice: If a database is I/O bound, we don't care about the extra CPU work or lock congestion of checking the clog. If it's CPU bound, the active pages that matter are in the buffer cache, and so are the hint bits for those pages. -- Heikki Linnakangas EnterpriseDB http://www.enterprisedb.com
On Fri, 2008-06-27 at 16:02 +0100, Simon Riggs wrote: > The patch splits into two parts: > * the machinery to *not* dirty a page when we set hints > * behaviour modifications now that we can tell the difference between > dirty and hinted pages > > Nobody has yet come up with any comments about the first half, which is > good. The second part is clearly where much debate will occur. I'm going > to literally split the patch into two, so we can get the machinery into > CVS and then fiddle and argue over the second part over next few months. The first "half" is actually quite large, but that makes it even more sensible to commit this part now. The enclosed patch introduces the machinery by which we might later optimise hint bit setting. It differentiates between hint bit setting and block dirtying, when the distinction can safely be made. It acts safely during VACUUM and correctly during checkpoint. In all other respects it emulates current behaviour. The actual tuning patch can be discussed later, probably at length. Later patches will be fairly small in comparison and so various people can fairly easily come up with their own favoured modifications for testing. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
Attachment
On Tue, Jul 1, 2008 at 4:13 AM, Simon Riggs <simon@2ndquadrant.com> wrote:
>
>
> The first "half" is actually quite large, but that makes it even more
> sensible to commit this part now.
>
> The enclosed patch introduces the machinery by which we might later
> optimise hint bit setting. It differentiates between hint bit setting
> and block dirtying, when the distinction can safely be made. It acts
> safely during VACUUM and correctly during checkpoint. In all other
> respects it emulates current behaviour.
>
As you yourself said, this patch mostly gets the machinery to count
hint bits in place and leaves the actual optimization for future. But
I think we should try at least one or two possible optimizations and
get some numbers before we jump and make substantial changes to the
code. Also that would help us in testing the patch for correctness and
performance.
For example, the following hunk seems buggy to me:
Index: src/backend/storage/buffer/bufmgr.c
===================================================================
RCS file: /home/sriggs/pg/REPOSITORY/pgsql/src/backend/storage/buffer/bufmgr.c,v
retrieving revision 1.232
diff -c -r1.232 bufmgr.c
*** src/backend/storage/buffer/bufmgr.c 12 Jun 2008 09:12:31 -0000 1.232
--- src/backend/storage/buffer/bufmgr.c 30 Jun 2008 22:17:20 -0000
***************
*** 1460,1473 ****
if (bufHdr->refcount == 0 && bufHdr->usage_count == 0)
result |= BUF_REUSABLE;
! else if (skip_recently_used)
{
/* Caller told us not to write recently-used buffers */
UnlockBufHdr(bufHdr);
return result;
}
! if (!(bufHdr->flags & BM_VALID) || !(bufHdr->flags & BM_DIRTY))
{
/* It's clean, so nothing to do */
UnlockBufHdr(bufHdr);
--- 1462,1477 ----
if (bufHdr->refcount == 0 && bufHdr->usage_count == 0)
result |= BUF_REUSABLE;
! else if (LRU_scan)
{
/* Caller told us not to write recently-used buffers */
UnlockBufHdr(bufHdr);
return result;
}
! if (!(bufHdr->flags & BM_VALID) ||
! !(bufHdr->flags & BM_DIRTY ||
! (LRU_scan && bufHdr->hint_count > 0)))
{
/* It's clean, so nothing to do */
UnlockBufHdr(bufHdr);
In the "if" condition above, we would throw away a buffer if the
hint_count is greater than zero, even if the buffer is dirty. This
doesn't seem correct to me, unless I am missing something obvious.
> The actual tuning patch can be discussed later, probably at length.
> Later patches will be fairly small in comparison and so various people
> can fairly easily come up with their own favoured modifications for
> testing.
>
>
I would suggest, let's have at least one tuning patch along with some
tests and numbers, before we go ahead and commit anything.
Thanks,
Pavan
--
Pavan Deolasee
EnterpriseDB http://www.enterprisedb.com
On Mon, 2008-07-21 at 11:36 +0530, Pavan Deolasee wrote: > I think we should try at least one or two possible optimizations and > get some numbers before we jump and make substantial changes to the > code. You know you're suggesting months of tests and further discussion. :-( I'll fix the bug, but I'm not doing any more on this now till feature freeze. It's the wrong time to chase mirages. Thanks for checking my logic. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
Simon Riggs <simon@2ndquadrant.com> writes:
> On Mon, 2008-07-21 at 11:36 +0530, Pavan Deolasee wrote:
>> I think we should try at least one or two possible optimizations and
>> get some numbers before we jump and make substantial changes to the
>> code.
> You know you're suggesting months of tests and further discussion. :-(
I agree with Pavan that we should have something that'd at least serve
as test scaffolding to verify that the framework patch is sane. The
test code needn't be anything we'd want to commit. It seems like
largely the same kind of issue as with your stats-hooks patch.
regards, tom lane
On Mon, 2008-07-21 at 16:33 -0400, Tom Lane wrote: > Simon Riggs <simon@2ndquadrant.com> writes: > > On Mon, 2008-07-21 at 11:36 +0530, Pavan Deolasee wrote: > >> I think we should try at least one or two possible optimizations and > >> get some numbers before we jump and make substantial changes to the > >> code. > > > You know you're suggesting months of tests and further discussion. :-( > > I agree with Pavan that we should have something that'd at least serve > as test scaffolding to verify that the framework patch is sane. The > test code needn't be anything we'd want to commit. The test code is/was there, in the sense that the patch was (supposed to) do exactly what it does now, just with extra code to keep track of hint counts. Probably the most important point is not yet covered: we don't keep any track of which blocks are dirtied solely for hint bits. We need to do this so we can measure the efficacy of *any* patch that seeks to improve the current situation. The best time to do this is in integration phase of release, so will do it then. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support
Simon Riggs wrote: > The first "half" is actually quite large, but that makes it even more > sensible to commit this part now. > > The enclosed patch introduces the machinery by which we might later > optimise hint bit setting. It differentiates between hint bit setting > and block dirtying, when the distinction can safely be made. It acts > safely during VACUUM and correctly during checkpoint. In all other > respects it emulates current behaviour. I think it makes sense to commit this patch now, per previous discussions on which we have agreed to make incremental changes. I think we should just get rid of the bogus changes Pavan identified. I'm just wondering if the change of usage_count from 16 to 8 bits was discussed and agreed? -- Alvaro Herrera http://www.CommandPrompt.com/ PostgreSQL Replication, Consulting, Custom Development, 24x7 support
Alvaro Herrera <alvherre@commandprompt.com> writes:
> I think it makes sense to commit this patch now, per previous
> discussions on which we have agreed to make incremental changes.
Yeah, but at the same time there is merit in the argument that the
proposed patch hasn't actually been proven to be usable for anything.
I would be a lot happier if there were even a trivial proof-of-concept
plugin example submitted with it, just to prove that there were no
showstopper problems in the plugin design, like failure to pass
essential information or not getting the locking straight.
> I'm just wondering if the change of usage_count from 16 to 8 bits was
> discussed and agreed?
Umm ... it was not, but given that we have logic in there to limit the
usage_count to 5 or so, it's hard to argue that there's a big problem.
I confess to not having read the patch in detail --- where did the other
8 bits go to?
regards, tom lane
On Sat, 2008-08-02 at 00:24 -0400, Tom Lane wrote: > Alvaro Herrera <alvherre@commandprompt.com> writes: > > I think it makes sense to commit this patch now, per previous > > discussions on which we have agreed to make incremental changes. > > Yeah, but at the same time there is merit in the argument that the > proposed patch hasn't actually been proven to be usable for anything. > I would be a lot happier if there were even a trivial proof-of-concept > plugin example submitted with it, just to prove that there were no > showstopper problems in the plugin design, like failure to pass > essential information or not getting the locking straight. Plugins were my other patch. I did originally submit a version with changes, but this patch was specifically a version with *no* external behaviour changes, to form a base from which various people's ideas might be explored. > > I'm just wondering if the change of usage_count from 16 to 8 bits was > > discussed and agreed? > > Umm ... it was not, but given that we have logic in there to limit the > usage_count to 5 or so, it's hard to argue that there's a big problem. It was discussed and it was Tom's suggestion to do this. I agreed! > I confess to not having read the patch in detail --- where did the other > 8 bits go to? Keeping track of the number of hints set on a block since last write. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support