Thread: random write in xlog?
Hi, folks,
I trace the running postgres, and I found that there are some randoms writes in xlog files!
To my impression, log file writing is always sequential, how come random writes happen?
Thanks in advance!
On Tue, Dec 7, 2010 at 2:06 PM, flyusa2010 fly <flyusa2010@gmail.com> wrote: > Hi, folks, > I trace the running postgres, and I found that there are some randoms writes > in xlog files! > To my impression, log file writing is always sequential, how come random > writes happen? > Thanks in advance! Just because it does an lseek doesn't mean it is "random". Anyway, the writes are logically sequentially, but not physically. If I remember correctly, it always writes out full blocks, even if the last part of the block has not yet been filled with new data. When the remainder gets filled, it then writes out the full block again, both the already written and the new part. Cheers, Jeff
>>>>> "JJ" == Jeff Janes <jeff.janes@gmail.com> writes: JJ> Anyway, the writes are logically sequentially, but not physically. JJ> If I remember correctly, it always writes out full blocks, even if JJ> the last part of the block has not yet been filled with new data. JJ> When the remainder gets filled, it then writes out the full block JJ> again, both the already written and the new part. What does that mean for use of a flash SSD for the xlog dir? Does the block writing mesh up well with the usage pattern a flash SSD needs to maximize lifespan? I'd love a dram ssd for pg_xlog and the journals for the other filesystems, but they cost too much. -JimC -- James Cloos <cloos@jhcloos.com> OpenPGP: 1024D/ED7DAEA6
On Wed, Dec 8, 2010 at 12:15 PM, James Cloos <cloos@jhcloos.com> wrote: >>>>>> "JJ" == Jeff Janes <jeff.janes@gmail.com> writes: > > JJ> Anyway, the writes are logically sequentially, but not physically. > JJ> If I remember correctly, it always writes out full blocks, even if > JJ> the last part of the block has not yet been filled with new data. > JJ> When the remainder gets filled, it then writes out the full block > JJ> again, both the already written and the new part. > > What does that mean for use of a flash SSD for the xlog dir? > > Does the block writing mesh up well with the usage pattern a flash > SSD needs to maximize lifespan? I think that SSD have a block size below which writing only part of the block has the same effect as writing the whole thing. And those block sizes are larger than 8K. So PG always writing 8K at a time is unlikely to make a difference than if it wrote a smaller amount. Cheers, Jeff
>>>>> "JJ" == Jeff Janes <jeff.janes@gmail.com> writes: JJ> So PG always writing 8K at a time is unlikely to make a difference JJ> than if it wrote a smaller amount. Ah. Somehow I was thinking of the xlog files' 16M filesize rather than the internal 8k block size.... If it is only writing 8k blocks then there is probably little hope of making efficient use of flash. -JimC -- James Cloos <cloos@jhcloos.com> OpenPGP: 1024D/ED7DAEA6