[linux-lvm] when bringing dm-cache online, consumes all memory and reboots

[Gionatan Danti]

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.

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Danti Gionatan
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