From: Gionatan Danti <g.danti@assyoma.it>
To: LVM general discussion and development <linux-lvm@redhat.com>
Cc: Roger Heflin <rogerheflin@gmail.com>,
Malin Bruland <malin.bruland@pm.me>
Subject: Re: [linux-lvm] lvconvert --uncache takes hours
Date: Thu, 02 Mar 2023 21:47:51 +0100 [thread overview]
Message-ID: <ff490b685777158d68c3fc966bd34c9e@assyoma.it> (raw)
In-Reply-To: <CAAMCDed4T5wF9VEZzkmLURS7vueLYhRPVZ+kJdGTYAtkMx81Bw@mail.gmail.com>
Il 2023-03-02 19:33 Roger Heflin ha scritto:
> On Thu, Mar 2, 2023 at 11:44 AM Gionatan Danti <g.danti@assyoma.it>
> wrote:
> It is a 100G cache over 16TB, so even if it flushes in order the may
> not be that close to each other (1 in 160).
Yes, but destaging in LBA order (albeit far apart) is much better than
in random order.
> Also if pieces are decided and added to the cached then the cache is
> not in order on the ssd and proper coalescing would require reading
> the entire cache and sorting the 3,000,000 location entries before
> starting the de-stage. And that complication of a de-stage is likely
> not been coded yet if I was just guessing, the de-stage starts at the
> beginning and continues to the end of the cache.
I would expect reordering and coalescing to happen in reasonably sized
window (ie: collect 64 MB of data, reorder and flush them). At the same
time, considering how lvmcache works, you are probably right: cached
chunks are going to be flushed as discovered (random order).
> Even coded though, if the you have enough blocks cached and if the
> blocks spread say one or 2 on each track it would break down to having
> to write a tiny bit on each track with seeks between mostly breaking
> down to the time required to simply read/write the HD end to end. At
> 150MB/sec (should be about the platter speed) that would take 3.5
> hours.
Which (apart being a totally worst outcome) would be way better than
what required for totally random IO.
Regards.
--
Danti Gionatan
Supporto Tecnico
Assyoma S.r.l. - www.assyoma.it
email: g.danti@assyoma.it - info@assyoma.it
GPG public key ID: FF5F32A8
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prev parent reply other threads:[~2023-03-02 20:48 UTC|newest]
Thread overview: 8+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-03-01 22:44 [linux-lvm] lvconvert --uncache takes hours Roy Sigurd Karlsbakk
2023-03-01 22:55 ` Demi Marie Obenour
2023-03-02 0:51 ` Roger Heflin
2023-03-02 8:33 ` Roy Sigurd Karlsbakk
2023-03-02 11:27 ` Roger Heflin
2023-03-02 17:34 ` Gionatan Danti
2023-03-02 18:33 ` Roger Heflin
2023-03-02 20:47 ` Gionatan Danti [this message]
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