[svlug] Sync command slowdown
james at linuxrebel.org
Mon Nov 24 20:20:43 PST 2008
On Monday 24 November 2008 18:17:15 Luke S Crawford wrote:
> James Sparenberg <james at linuxrebel.org> writes:
> > I've got 4 new DB servers exhibiting a rather problematic set of
> > actions with disk sync. Running the sync command (as is done when
> > are rotated.) takes 4-6 seconds (not micro, full seconds) Other
> > this these boxes are running 10x faster than the servers they
> > and making our customers much happier.
> Are you using ext3? (or something else that does write-back caching?)
> I saw something very similar the other night when I was setting up my
> ridiculous servers (I switched to 32GB ram/dual socket quad-core
> for my Xen hosting setup.) - See, normally I limit the Dom0 to
> 512M or a gigabyte of ram, but these were new servers and I was just
> with them in the garage. I hadn't even installed the xen kernel yet,
> the OS instance had a whole 32GB ram to play with. I ran some disk
> a dd command, bonnie, - the system screamed. Sync, on the other
> took north of 120 seconds to return.
> but on another console, iostat showed that my sata disks were still
> away, even though the disk tests finished some time ago. As far as I
> tell, it was just the expected ext3 write caching, just, you know,
> it takes a long time to flush 32GB of data to SATA disks.
> But yeah, that'd be my guess; you just have lots of dirty buffers.
> means that you can buffer more, and disks are still pretty slow.
Yes I am using ext3 and in the process I also found this at kerneltrap.
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