From: Pavel Goran <via-bcache@pvgoran.name>
To: Nix <nix@esperi.org.uk>
Cc: Coly Li <colyli@suse.de>,
linux-bcache@vger.kernel.org, linux-block@vger.kernel.org
Subject: Re[2]: [PATCH v3 00/13] bcache: device failure handling improvement
Date: Fri, 26 Jan 2018 07:15:41 +0300 [thread overview]
Message-ID: <355593907.20180126071541@pvgoran.name> (raw)
In-Reply-To: <874ln9hilm.fsf@esperi.org.uk>
Hello Nix,
Thursday, January 25, 2018, 9:57:25 PM, you wrote:
> On 25 Jan 2018, Pavel Goran told this:
>> Hello Nix,
>>
>> Thursday, January 25, 2018, 1:23:19 AM, you wrote:
>>
>>> This feels wrong to me. If a cache device is writethrough, the cache is
>>> a pure optimization: having such a device fail should not lead to I/O
>>> failures of any sort, but should only flip the cache device to 'none' so
>>> that writes to the backing store simply don't get cached any more.
>>
>>> Anything else leads to a reliability reduction, since in the end cache
>>> devices *will* fail.
>>
>> It's one of those choices: "if something can't work as intended, should it be
>> allowed to work at all?"
> Given that the only difference between a bcache with a writearound cache
> and a bcache with no cache is performance... is it really ever going to
> beneficial to users to have a working system suddenly start throwing
> write errors and probably become instantly nonfunctional because a
> cache device has worn out, when it is perfectly possible to just
> automatically dissociate the failed cache and slow down a bit?
> I would suggest that no user would ever want the former behaviour, since
> it amounts to behaviour that worsens a slight slowdown into a complete
> cessation of service (in effect, an infinite "slowdown"). Is it better
> to have a system working correctly but more slowly than before, or one
> that without warning stops working entirely? Is this really even in
> question?!
Well, there is the "Fail-Fast" principle [1] and all that. For a home user
(which is my case, for example), this approach doesn't make much sense.
However, large-scale users, like cloud providers, can have a different point
of view.
It's just a speculation on my part, but consider a bunch of bcache devices
that serve as parts of a RAID6 array. It may be desirable to deactivate the
bcache device that lost its caching capabilities, so that (1) the array would
not slow down, (2) the array would report its degraded state to
administrators.
Anyway, probably the author of this patch could explain it better. Maybe I
completely misunderstand the intention.
Pavel Goran
[1] https://en.wikipedia.org/wiki/Fail-fast
next prev parent reply other threads:[~2018-01-26 4:15 UTC|newest]
Thread overview: 32+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-01-14 14:42 [PATCH v3 00/13] bcache: device failure handling improvement Coly Li
2018-01-14 14:42 ` [PATCH v3 01/13] bcache: set writeback_rate_update_seconds in range [1, 60] seconds Coly Li
2018-01-16 9:03 ` Hannes Reinecke
2018-01-14 14:42 ` [PATCH v3 02/13] bcache: properly set task state in bch_writeback_thread() Coly Li
2018-01-16 9:02 ` Hannes Reinecke
2018-01-14 14:42 ` [PATCH v3 03/13] bcache: set task properly in allocator_wait() Coly Li
2018-01-16 9:05 ` Hannes Reinecke
2018-01-16 9:29 ` Coly Li
2018-01-14 14:42 ` [PATCH v3 04/13] bcache: fix cached_dev->count usage for bch_cache_set_error() Coly Li
2018-01-14 14:42 ` [PATCH v3 05/13] bcache: quit dc->writeback_thread when BCACHE_DEV_DETACHING is set Coly Li
2018-01-16 9:11 ` Hannes Reinecke
2018-01-26 6:21 ` Coly Li
2018-01-14 14:42 ` [PATCH v3 06/13] bcache: stop dc->writeback_rate_update properly Coly Li
2018-01-14 14:42 ` [PATCH v3 07/13] bcache: set error_limit correctly Coly Li
2018-01-14 14:42 ` [PATCH v3 08/13] bcache: add CACHE_SET_IO_DISABLE to struct cache_set flags Coly Li
2018-01-14 14:42 ` [PATCH v3 09/13] bcache: stop all attached bcache devices for a retired cache set Coly Li
2018-01-14 14:42 ` [PATCH v3 10/13] bcache: fix inaccurate io state for detached bcache devices Coly Li
2018-01-16 9:27 ` Hannes Reinecke
2018-01-14 14:42 ` [PATCH v3 11/13] bcache: add backing_request_endio() for bi_end_io of attached backing device I/O Coly Li
2018-01-16 9:28 ` Hannes Reinecke
2018-01-14 14:42 ` [PATCH v3 12/13] bcache: add io_disable to struct cached_dev Coly Li
2018-01-16 9:32 ` Hannes Reinecke
2018-01-14 14:42 ` [PATCH v3 13/13] bcache: stop bcache device when backing device is offline Coly Li
2018-01-16 9:33 ` Hannes Reinecke
2018-01-24 22:23 ` [PATCH v3 00/13] bcache: device failure handling improvement Nix
2018-01-25 3:35 ` Re[2]: " Pavel Goran
2018-01-25 18:57 ` Nix
2018-01-26 4:15 ` Pavel Goran [this message]
2018-01-26 4:56 ` Coly Li
2018-01-26 5:51 ` Michael Lyle
2018-01-26 6:23 ` Coly Li
2018-02-16 12:11 ` Nix
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