On Tue, 09 Mar 2010 08:00:50 +0100, Greg Smith <greg@2ndquadrant.com>
wrote:
> Scott Carey wrote:
>> For high sequential throughput, nothing is as optimized as XFS on Linux
>> yet. It has weaknesses elsewhere however.
>>
When files are extended one page at a time (as postgres does)
fragmentation can be pretty high on some filesystems (ext3, but NTFS is
the absolute worst) if several files (indexes + table) grow
simultaneously. XFS has delayed allocation which really helps.
> I'm curious what you feel those weaknesses are.
Handling lots of small files, especially deleting them, is really slow on
XFS.
Databases don't care about that.
There is also the dark side of delayed allocation : if your application is
broken, it will manifest itself very painfully. Since XFS keeps a lot of
unwritten stuff in the buffers, an app that doesn't fsync correctly can
lose lots of data if you don't have a UPS.
Fortunately, postgres handles fsync like it should be.
A word of advice though : a few years ago, we lost a few terabytes on XFS
(after that, restoring from backup was quite slow !) because a faulty SCSI
cable crashed the server, then crashed it again during xfsrepair. So if
you do xfsrepair on a suspicious system, please image the disks first.
