[linux-lvm] Unable to un-cache logical volume when chunk size is over 1MiB

[Ryan Launchbury]

Hi Zdenek and Gionatan,

Thanks for your reply's.
Something else of note: on systems which are unable to flush the cache,
data is still being written to the origin LV somehow, because there is
200TB of data in the LV, but the cache is only 1.8TB, so somehow it is
working. However when running any commands to flush the cache, or uncache,
it seems unable to.

What sort of admin work needs to be done/can be done to force the flush and
remove the cache?
I've tried the cleaner policy, however, it doesn't seem to be flushing
anything.

In testing, forcibly removing the cache, via editing the LVM config file
has caused extensive XFS filesystem corruption, even when backing up the
metadata first and restoring after the cache device is missing. Any advice
on how to safely uncache the volume would be massively appreciated.

Please let me know if you need any more logs or data.
Best regards,
Ryan


On Sat, Jun 23, 2018 at 11:09 AM Gionatan Danti <g.danti at assyoma.it> wrote:

> Il 22-06-2018 22:07 Zdenek Kabelac ha scritto:
> > When cache will experience write error - it will become invalidate and
> > will
> > need to be dropped - but this thing is not automated ATM - so admin
> > works is needed to handle this task.
>
> So, if a writethrough cache experience write errors but the
> administrator is not able to immediately intervene to drop the cache,
> what problem can arise? Stale reads? Slow performance?
>
> What about cache *read* error? Is the read simply redirected to the
> underlying slow/main volume?
>
> Thanks.
>
> --
> Danti Gionatan
> Supporto Tecnico
> Assyoma S.r.l. - www.assyoma.it
> email: g.danti at assyoma.it - info at assyoma.it
> GPG public key ID: FF5F32A8
>
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