Date: Sat, 21 Jan 2006 21:58:19 +0100 (CET) From: Oliver Fromme <olli@lurza.secnetix.de> To: freebsd-fs@FreeBSD.ORG Subject: Re: can snapshots become corrupted ? Is fsck'ing /dev/md0 sensible ? Message-ID: <200601212058.k0LKwJ9Q059337@lurza.secnetix.de> In-Reply-To: <20060121172644.19851.qmail@web50914.mail.yahoo.com>
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Joe Schmoe <non_secure@yahoo.com> wrote: > On Sat, 21 Jan 2006, Oliver Fromme wrote: > > Yes, the snapshot is probably still "dirty". But it > > shouldn't matter, because you can only mount it read-only > > anyway. > > Ok, yes, the snapshot can only be mounted read-only - > this is true. However, the snapshot itself (whether > mounted or not) is continually being changed as files > are being changed or deleted on the filesystem in > question. So if the snapshot is corrupt, and I start > making changes/deletions on the (now clean) > filesystem, then wouldn't there be problems ? That question would have to be answered by a snapshots expert. But I guess you're right, there are probably problems. > Ok, understood. However, once I do a full and > successful fsck on that filesystem, it is completely > safe again, regardless of how long or how often I ran > it while it was dirty, right ? Right, provided the dirtyness was only soft-updates related and the disks were reliable. > [...] > The second question I need to ask is, when I am > rsyncing this filesystem to a remote host, why is it > not a read-only operation ? My rsync process, because > this filesystem was the _source_, and not the > destination, should not have written anything to this > filesystem. However, it succeeded when the fs was > read-only (softupdates were off) and it failed when > the filesystem was read-write (softupdates on). Is > there some kind of manipulation of the source > filesystem that rsync does that would be equal to a > lot of writing to the source disk ? When you read from a file, its atime (access time) is scheduled for an update (unless the FS is mounted with the "noatime" flag). That's a write operation. So when you rsync a lot of files, quite a lot of meta data updates can pile up. That happens only if the FS is mounted read-write, of course. > It is my understanding that soft-updates only deal > with writes to the disk That's right. Best regards Oliver -- Oliver Fromme, secnetix GmbH & Co. KG, Marktplatz 29, 85567 Grafing Dienstleistungen mit Schwerpunkt FreeBSD: http://www.secnetix.de/bsd Any opinions expressed in this message may be personal to the author and may not necessarily reflect the opinions of secnetix in any way. "Being really good at C++ is like being really good at using rocks to sharpen sticks." -- Thant Tessman
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