Francisco Reyes wrote:
> Yeb Havinga writes:
>
>> controllers. Also, I am not sure if it is wise to put the WAL on the
>> same logical disk as the indexes,
>
> If I only have two controllers would it then be better to put WAL on
> the first along with all the data and the indexes on the external?
> Specially since the external enclosure will have 15K rpm vs 10K rpm in
> the internal.
It sounds like you're going to create a single logical unit / raid array
on each of the controllers. Depending on the number of disks, this is a
bad idea because if you'd read/write data sequentially, all drive heads
will be aligned to roughly the same place ont the disks. If another
process wants to read/write as well, this will interfere and bring down
both iops and mbps. However, with three concurrent users.. hmm but then
again, queries will scan multiple tables/indexes so there will be mixed
io to several locations. What would be interesting it so see what the
mbps maximum of a single controller is. Then calculate how much disks
are needed to feed that, which would give a figure for number of disks
per logical unit.
The challenge with having a few logical units / raid arrays available,
is how to divide the data over it (with tablespaces) What is good for
your physical data depends on the schema and queries that are most
important. For 2 relations and 2 indexes and 4 arrays, it would be
clear. There's not much to say anything general here, except: do not mix
table or index data with the wal. In other words: if you could make a
separate raid array for the wal (2 disk raid1 probably good enough),
that would be ok and doesn't matter on which controller or enclosure it
happens, because io to disk is not mixed with the data io.
>
> Thanks for pointing that out.
> With any luck I will actually be able to do some tests for the new
> hardware. The curernt one I literaly did a few hours stress test and
> had to put in production right away.
I've heard that before ;-) If you do get around to do some tests, I'm
interested in the results / hard numbers.
regards,
Yeb Havinga
