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 18:34:04 +0100 [thread overview]
Message-ID: <17a0cca9c38c6eb8384275f68a2b465f@assyoma.it> (raw)
In-Reply-To: <CAAMCDeftn7gLN8s3EMFer+jJ95dRkdpR4007hDMqNNhjrbGDJA@mail.gmail.com>
Il 2023-03-02 01:51 Roger Heflin ha scritto:
> A spinning raid6 array is slow on writes (see raid6 write penalty).
> Because of that the array can only do about 100 write operattions/sec.
True. But does flushing cached data really proceed in random LBA order
(as seen by HDDs), rather than trying to coalesce writes in linear
fashion?
> If the disk is doing other work then it only has the extra capacity so
> it could destage slower.
>
> A lot depends on how big each chunk is. The lvmcache indicates the
> smallest chunksize is 32k.
>
> 100G / 32k = 3 million, and at 100seeks/sec that comes to at least an
> hour.
You are off an order of magnitude: 3 millions IOP at 100 IOPs means
~30000s, so about 9 hours.
> Lvm bookkeeping has to also be written to the spinning disks I would
> think, so 2 hours if the array were idle.
>
> Throw in a 50% baseload on the disks and you get 4 hours.
>
> Hours is reasonable.
If flushing happens in random disk order, than yes, you are bound to
wait several hours indeed.
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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next prev parent reply other threads:[~2023-03-02 17:44 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 [this message]
2023-03-02 18:33 ` Roger Heflin
2023-03-02 20:47 ` Gionatan Danti
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