On 4/13/21 4:07 AM, Dan van der Ster wrote:
On Tue, Apr 13, 2021 at 9:00 AM Wido den Hollander <wido@xxxxxxxx> wrote:
On 4/12/21 5:46 PM, Dan van der Ster wrote:
Hi all,
bdev_enable_discard has been in ceph for several major releases now
but it is still off by default.
Did anyone try it recently -- is it safe to use? And do you have perf
numbers before and after enabling?
I have done so on SATA SSDs in a few cases and: it worked
Did I notice a real difference? Not really.
Thanks, I've enabled it on a test box and am draining data to check
that it doesn't crash anything.
It's highly debated if this still makes a difference with modern flash
devices. I don't think there is a real conclusion if you still need to
trim/discard blocks.
Do you happen to have any more info on these debates? As you know we
have seen major performance issues on hypervisors that are not running
a periodic fstrim; we use similar or identical SATA ssds for HV local
storage and our block.db's. If it doesn't hurt anything, why wouldn't
we enable it by default?
There's some good discussion in the original PR:
https://github.com/ceph/ceph/pull/14727
I suspect that the primary concerns for enabling it by default are two
fold: (1) the issue of having to maintain a blocklist for buggy firmware
implementations (2) Even "good" firmware can potentially see slowdowns
with bursts of trim commands due to needing to update the FTL metadata
per this comment:
https://github.com/ceph/ceph/pull/14727#issuecomment-342399578
The original issue of how to decide between online discard, periodic
bulk discard, or no discard is still an issue imho. I think we probably
need to get more feedback from people with real large deployments (hint
hint :D) before we enable online discard by default.
Mark
Cheers, Dan
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