From mboxrd@z Thu Jan 1 00:00:00 1970 From: Moshe Yudkowsky Subject: Re: RAID needs more to survive a power hit, different /boot layout for example (was Re: draft howto on making raids for surviving a disk crash) Date: Mon, 04 Feb 2008 04:58:23 -0600 Message-ID: <47A6EFCF.9080906@pobox.com> References: <47A612BE.5050707@pobox.com> <47A623EE.4050305@msgid.tls.msk.ru> <47A62A17.70101@pobox.com> <47A6DA81.3030008@msgid.tls.msk.ru> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Return-path: In-Reply-To: <47A6DA81.3030008@msgid.tls.msk.ru> Sender: linux-raid-owner@vger.kernel.org To: Michael Tokarev Cc: linux-raid@vger.kernel.org List-Id: linux-raid.ids Michael Tokarev wrote: > Moshe Yudkowsky wrote: > [] >> But that's *exactly* what I have -- well, 5GB -- and which failed. I've >> modified /etc/fstab system to use data=journal (even on root, which I >> thought wasn't supposed to work without a grub option!) and I can >> power-cycle the system and bring it up reliably afterwards. > > Note also that data=journal effectively doubles the write time. > It's a bit faster for small writes (because all writes are first > done into the journal, i.e. into the same place, so no seeking > is needed), but for larger writes, the journal will become full > and data found in it needs to be written to proper place, to free > space for new data. Here, if you'll continue writing, you will > have more than 2x speed degradation, because of a) double writes, > and b) more seeking. The alternative seems to be that portions of the / file system won't mount because the file system is corrupted on a crash while writing. If I'm reading the man pages, Wikis, READMEs and mailing lists correctly -- not necessarily the case -- the ext3 file system uses the equivalent of data=journal as a default. The question then becomes what data scheme to use with reiserfs on the remainder of the file system, the /usr, /var, /home, and others. If they can recover on a reboot sing fsck and the default configuration of resierfs, then I have no problem using them. But my understanding is that data can be destroyed or lost or destroyed if there's a crash on a write; then there's little point in running a RAID system that can collect corrupt data. Another way to phrase this: unless you're running data-center grade hardware and have absolute confidence in your UPS, you should use data=journal for reiserfs and perhaps avoid XFS entirely. -- Moshe Yudkowsky * moshe@pobox.com * www.pobox.com/~moshe "Right in the middle of a large field where there had never been a trench was a shell hole... 8 feet deep by 15 across. On the edge of it was a dead... rat not over twice the size of a mouse. No wonder the war costs so much." Col. George Patton