Mailinglist Archive: opensuse (1470 mails)

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Re: [opensuse] experiences with bache / logic of caching
  • From: Greg Freemyer <greg.freemyer@xxxxxxxxx>
  • Date: Wed, 17 Feb 2016 11:51:05 -0500
  • Message-id: <CAGpXXZJr0DKPB3CF+Or24H2vVsLq=-J3x+cLjxY2VBuEbW5SOQ@mail.gmail.com>
On Wed, Feb 17, 2016 at 7:03 AM, Per Jessen <per@xxxxxxxxxxxx> wrote:
Andrei Borzenkov wrote:

On Wed, Feb 17, 2016 at 2:28 PM, Per Jessen <per@xxxxxxxxxxxx> wrote:

I'll have to check, but I don't think adding memory will do much. The
filesystems cache is not used because there are no filesystems in use -
the raw disk space is handed out over iSCSI, just as Greg described.


Caches are per-block device, so unless you are using devices for
direct IO they should still benefit from RAM caches.

Each storage server has 4Gb, expandable to 32Gb afaict - but ietd does use
direct IO. (type=blockio). I think I need to check up on fileio vs.
blockio. Adding some more RAM is a lot easier than adding a bcache.

The purpose of O_DIRECT is to move the cache handling from kernel
space to userspace. Does ietd not implement a cache in user space?

If so, you should verify that FUA, DPO, CACHE_FLUSH commands do /
don't work and that your filesystem is able to handle any false
successes those commands trigger.

XFS in particular has advice as to how to address features falsely
reporting success.

note that if all those features are well implemented in ietd, then
letting your filesystem leverage them will improve performance (at
least theoretically. XFS has worked hard at being well performing even
if FUA in particular is not reliable, but it needs to be told.

Greg
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