linux-ext4 - Re: fsync() errors is unsafe and risks data loss

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Date:   Thu, 19 Apr 2018 01:39:04 -0700
From:   Christoph Hellwig <hch@...radead.org>
To:     "J. Bruce Fields" <bfields@...ldses.org>
Cc:     Martin Steigerwald <martin@...htvoll.de>,
        "Theodore Y. Ts'o" <tytso@....edu>,
        "Joshua D. Drake" <jd@...mandprompt.com>,
        linux-ext4@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: Re: fsync() errors is unsafe and risks data loss

On Wed, Apr 18, 2018 at 12:52:19PM -0400, J. Bruce Fields wrote:
> > Theodore Y. Ts'o - 10.04.18, 20:43:
> > > First of all, what storage devices will do when they hit an exception
> > > condition is quite non-deterministic.  For example, the vast majority
> > > of SSD's are not power fail certified.  What this means is that if
> > > they suffer a power drop while they are doing a GC, it is quite
> > > possible for data written six months ago to be lost as a result.  The
> > > LBA could potentialy be far, far away from any LBA's that were
> > > recently written, and there could have been multiple CACHE FLUSH
> > > operations in the since the LBA in question was last written six
> > > months ago.  No matter; for a consumer-grade SSD, it's possible for
> > > that LBA to be trashed after an unexpected power drop.
> 
> Pointers to documentation or papers or anything?  The only google
> results I can find for "power fail certified" are your posts.
> 
> I've always been confused by SSD power-loss protection, as nobody seems
> completely clear whether it's a safety or a performance feature.

Devices from reputable vendors should always be power fail safe, bugs
notwithstanding.  What power-loss protection in marketing slides usually
means is that an SSD has a non-volatile write cache.  That is once a
write is ACKed data is persisted and no additional cache flush needs to
be sent.  This is a feature only available in expensive eterprise SSDs
as the required capacitors are expensive.  Cheaper consumer or boot
driver SSDs have a volatile write cache, that is we need to do a
separate cache flush to persist data (REQ_OP_FLUSH in Linux).  But
a reasonable implementation of those still won't corrupt previously
written data, they will just lose the volatile write cache that hasn't
been flushed.  Occasional bugs, bad actors or other issues might still
happen.

> 
> --b.
---end quoted text---