John Stoffel schrieb:
Norman> What you should be able to do with software raid1 is the
Norman> following: Stop the raid, mount both underlying devices
Norman> instead of the raid device, but of course READ ONLY. Both
Norman> contain the complete data and filesystem, and in addition to
Norman> that the md superblock at the end. Both should be identical
Norman> copies of that. Thus, you do not have to resync
Norman> afterwards. You then can backup the one disk while serving the
Norman> web server from the other. When you are done, unmount,
Norman> assemble the raid, mount it and go on.
Umm... so what you're proposing would mean that he would have to stop
his application, make sure nothing is accessing that volume, and then
restart the application?
Somehow I don't see that happening.
Why do you feel that you have to stop and umount the volume for the
splitting off of a read-only mirror pair for backups?
John
All this "pulling a disk" sounded to me like a resync, and with that
much data and slow access this resync would take ages.
And as far as I remember, there was a suggestion with stopping or
interrupting applications anyway, was there not? Unfortunately, I have
thrown most of the posts away.
If you keep the volume running, ok - it should still work from the data
integrity point of view. But if you keep using the data from a
functional raid1 (/dev/md1 on /data), you cannot disentangle the access,
since the raid1 will read from both mirrors. But what was desired was a
faster backup and being able to continue serving the data.
If one disk (without the read-balancing of raid1) would provide enough
treoughput for the webserver, one could even do the following:
mount /dev/md1 /writeaccess
mount -r /dev/hda /data
mount -r /dev/hdc /backup
So write from /weiteacces (mirrored to both drives) and read from one
for the webserver (statically) and from the other one for the backup.
Could that work?
Norman.
--
Norman Schmidt Institut fuer Physikal. u. Theoret. Chemie
Dipl.-Chem. Univ. Friedrich-Alexander-Universitaet
schmidt@xxxxxxx Erlangen-Nuernberg
+49 9131 852 7321 IT-Systembetreuer Physikalische Chemie
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