Re: SSD usage for bcache - Read and Writeback

[Emmanuel Florac <eflorac@intellique.com>]

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Le Thu, 14 Sep 2017 11:46:11 -0300
FERNANDO FREDIANI <fernando.frediani@upx.com> écrivait:

> Not really mandatory but optional.
> 
> From bcache documentation:
> 
> "In the event of a data IO error on the flash it will try to recover
> by reading from disk or invalidating cache entries.  For
> unrecoverable errors (meta data or dirty data), caching is
> automatically disabled; if dirty data was present in the cache it
> first disables writeback caching and waits for all dirty data to be
> flushed."

If the cache is dirty and has failed, your data is lost or corrupted
all the same. The way a SSD fails is usually one second it works, the
next second it's as dead as a rock. If you're using it as a write
cache, whatever amount of data is dirty is lost. If it's 8 GB, you've
lost your last 8 GB of writes, often very important filesystem metadata
and/or database journal, i.e. your data is completely hosed.

Don't use SSDs as write cache without RAID-1 (or use very expensive,
redundant NVMe SSDs only), unless your data isn't very important.

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