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From: Konstantin Osipov <kostja@tarantool.org>
To: Vladimir Davydov <vdavydov.dev@gmail.com>
Cc: tarantool-patches@freelists.org
Subject: Re: [PATCH] xlog: fix fallocate vs read race
Date: Fri, 14 Dec 2018 14:07:41 +0300	[thread overview]
Message-ID: <20181214110741.GA7745@chai> (raw)
In-Reply-To: <8548a4bd8439a1e4a7f78ff37216c170c61a33c3.1544783335.git.vdavydov.dev@gmail.com>

* Vladimir Davydov <vdavydov.dev@gmail.com> [18/12/14 13:30]:
> posix_fallocate(), which is used for preallocating disk space for WAL
> files, increases the file size and fills the allocated space with zeros.
> The problem is a WAL file may be read by a relay thread at the same time
> it is written to. We try to handle the zeroed space in xlog_cursor (see
> xlog_cursor_next_tx()), however this turns out to be not enough, because
> transactions are written not atomically so it may occur that a writer
> writes half a transaction when a reader reads it. Without fallocate, the
> reader would stop at EOF until the rest of the transaction is written,
> but with fallocate it reads zeroes instead and thinks that the xlog file
> is corrupted while actually it is not.

You should use check_program_runs() not check_symbol_exists and
avoid checks at runtime. 

Please push this patch and then proceed to fix it according to the
review.
> 
> Fix this issue by using fallocate() with FALLOC_FL_KEEP_SIZE flag
> instead of posix_fallocate(). With the flag fallocate() won't increase
> the file size, it will only allocate disk space beyond EOF.
> 
> The test will be added shortly.

-- 
Konstantin Osipov, Moscow, Russia, +7 903 626 22 32
http://tarantool.io - www.twitter.com/kostja_osipov

  reply	other threads:[~2018-12-14 11:07 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-12-14 10:29 Vladimir Davydov
2018-12-14 11:07 ` Konstantin Osipov [this message]
2018-12-14 11:12   ` Vladimir Davydov
2018-12-14 12:04 ` Vladimir Davydov
2018-12-14 12:30 ` Vladimir Davydov

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