From mboxrd@z Thu Jan 1 00:00:00 1970 From: Tejun Heo Subject: Re: when are buffers/caches flushed? Date: Thu, 18 Aug 2005 20:59:44 +0900 Message-ID: <43047830.4060705@gmail.com> References: <43046817.803@imc-berlin.de> <43046FD2.4080401@gmail.com> <43047229.7010007@imc-berlin.de> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: Received: from wproxy.gmail.com ([64.233.184.198]:58475 "EHLO wproxy.gmail.com") by vger.kernel.org with ESMTP id S932151AbVHRL76 (ORCPT ); Thu, 18 Aug 2005 07:59:58 -0400 Received: by wproxy.gmail.com with SMTP id i20so340658wra for ; Thu, 18 Aug 2005 04:59:50 -0700 (PDT) In-Reply-To: <43047229.7010007@imc-berlin.de> Sender: linux-ide-owner@vger.kernel.org List-Id: linux-ide@vger.kernel.org To: Steven Scholz , Bartlomiej Zolnierkiewicz Cc: linux-ide@vger.kernel.org Steven Scholz wrote: > Tejun Heo wrote: > >> Steven Scholz wrote: >> >>> Hi there, >>> >>> According to an (old) man page of sync(2) >>> >>> According to the standard specification (e.g., SVID), >>> sync() schedules the writes, but may return before the >>> actual writing is done. However, since version 1.3.20 >>> Linux does actually wait. (This still does not guarantee >>> data integrity: modern disks have large caches.) >>> >>> How about recent kernels? Does sync() block until buffers are flushed? >>> How can I find out if the disk caches are actually flushed? >>> >>> I want to make sure that all data is flushed to my disk drive before >>> powering down the system. >>> >>> Thanks. >>> >> >> All disk caches are flushed before shutdown via the following path. >> >> kernel/sys.c::sys_reboot() >> kernel/drivers/base/power/shutdown.c::device_shutdown() >> driver specific ->shutdown callback, for ide disks, the path is >> drivers/ide/ide-disk.c::ide_device_shutdown() >> drivers/ide/ide-disk.c::ide_cacheflush_p() >> drivers/ide/ide-disk.c::do_idedisk_flushcache() >> >> And, AFAIK, sync() doesn't flush disk caches. > > > How about umount? > [CC'ing Bartlomiej (Hi!)] Hmmm, umount doesn't. I think maybe adding cache flushing to sync and umount can be helpful. And one more thing, ide-disk doesn't flush cache when shutting down. It flushes only when rebooting. from ide_device_shutdown()... #ifdef CONFIG_ALPHA /* On Alpha, halt(8) doesn't actually turn the machine off, it puts you into the sort of firmware monitor. Typically, it's used to boot another kernel image, so it's not much different from reboot(8). Therefore, we don't need to spin down the disk in this case, especially since Alpha firmware doesn't handle disks in standby mode properly. On the other hand, it's reasonably safe to turn the power off when the shutdown process reaches the firmware prompt, as the firmware initialization takes rather long time - at least 10 seconds, which should be sufficient for the disk to expire its write cache. */ if (system_state != SYSTEM_POWER_OFF) { #else if (system_state == SYSTEM_RESTART) { #endif ide_cacheflush_p(drive); return; } It seems that the if clause are there due to some historic thing. The comment suggests that the body in the if clause spun down the drive previously, and we needed the if to avoid spinning down unnecessarily, and the if is left there after we changed the body to flush cache. Bartlomiej, am I missing something? -- tejun