From: Gionatan Danti <firstname.lastname@example.org> To: Zdenek Kabelac <email@example.com> Cc: Mcdermott <firstname.lastname@example.org>, Scott, LVM general discussion and development <email@example.com> Subject: Re: [linux-lvm] when bringing dm-cache online, consumes all memory and reboots Date: Tue, 24 Mar 2020 23:35:12 +0100 [thread overview] Message-ID: <firstname.lastname@example.org> (raw) In-Reply-To: <email@example.com> Il 2020-03-24 16:09 Zdenek Kabelac ha scritto: > In past we had problem that when users have been using huge chunk size, > and small 'migration_threashold' - the cache was unable to demote > chunks > from the cache to the origin device (the size of 'required' data for > demotion were bigger then what has been allowed by threshold). > > So lvm2/libdm implemented protection to always set at least 8 chunks > is the bare minimum. > > Now we face clearly the problem from 'the other side' - users have way > too big chunks (we've seen users with 128M chunks) - and so threshold > is set to 1G > and users are facing serious bottleneck on the cache side doing to many > promotions/demotions. > > We will likely fix this by setting max chunk size somewhere around > 2MiB. Thanks for the explanation. Maybe is a naive proposal, but can't you simply set migration_threshold equal to a single chunk for >2M sized chunks, and 8 chunks for smaller ones? > Yeah - if you read only 'directory' metadata structures - it's > perfectly OK with caching, if you do a full data read of the whole > storage it's not going to help (which is what I've meant). > > The main message is - cache is there to accelerate often read disk > blocks. > If there is no hotspot and disk is mostly read over the whole address > space equally there will be no big benefit of cache usage. > > However main message should be - user should think about sizes of > caching > devices and its implications - there is no universal setting that fits > best all users. Sure. -- Danti Gionatan Supporto Tecnico Assyoma S.r.l. - www.assyoma.it  email: firstname.lastname@example.org - email@example.com GPG public key ID: FF5F32A8
next prev parent reply other threads:[~2020-03-24 22:35 UTC|newest] Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top 2020-03-22 17:57 Scott Mcdermott 2020-03-23 8:26 ` Joe Thornber 2020-03-23 9:57 ` Zdenek Kabelac 2020-03-23 16:26 ` John Stoffel 2020-03-23 22:02 ` Scott Mcdermott 2020-03-24 9:43 ` Zdenek Kabelac 2020-03-24 11:37 ` Gionatan Danti 2020-03-24 15:09 ` Zdenek Kabelac 2020-03-24 22:35 ` Gionatan Danti [this message] 2020-03-25 8:55 ` Zdenek Kabelac 2020-03-23 21:35 ` Scott Mcdermott
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