From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Date: Fri, 14 Dec 2018 14:07:41 +0300 From: Konstantin Osipov Subject: Re: [PATCH] xlog: fix fallocate vs read race Message-ID: <20181214110741.GA7745@chai> References: <8548a4bd8439a1e4a7f78ff37216c170c61a33c3.1544783335.git.vdavydov.dev@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <8548a4bd8439a1e4a7f78ff37216c170c61a33c3.1544783335.git.vdavydov.dev@gmail.com> To: Vladimir Davydov Cc: tarantool-patches@freelists.org List-ID: * Vladimir Davydov [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