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ru.linux
- RU.LINUX ---------------------------------------------------------------------
From : Mike Novikoff 2:5020/133.73 26 Jul 2002 21:07:18
To : Valentin Nechayev
Subject : Copy Linux -> new hdd
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VN> Стандартные гнутые dump/restore могут безнадежно лажаться на живых
VN> дисках. См. lkml, даже его величество Линус соизволил признать, что
VN> dump/restore сейчас вреден.
Кстати, вот. Было даже в этой эхе. После прочтения я вообще удалил dump
со своей машины, а текст сохранил на память.
(И, кстати, куда теперь пропали эти постинги? Удобно было).
===========================================================================
From : Sergey Lentsov 2:4615/71.10 03.05.2001 17:11:10## To : All
Subj : URL: http://lwn.net/2001/0503/kernel.php3
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Trashing your filesystem with dump. It has been known for a very long
time that using dump to back up live filesystems can result in corrupt
backups. It turns out that, with Linux kernels through 2.4.4, dumping
a live filesystem has the potential to corrupt the filesystem in
place, even if the dump process has no write access.
Alexander Viro [20]reported the bug which makes this possible. It can
happen only on SMP systems, and is not easy to trigger, but it is
there. Essentially, if the filesystem allocates a new metadata block
for the filesystem, and a separate process reads that block at the
wrong time, the wrong data will be written back to disk. The fix is
relatively straightforward, and has [21]already been incorporated into
2.4.5pre1.
Linus [22]pointed out an interesting little fact as part of this
discussion: dump will not work correctly on 2.4-based systems in any
case. The filesystem keeps quite a bit of useful information in the
page cache - and will do so even more in the future. dump, however,
works with the raw device, which deals with the buffer cache instead.
The two are not always synchronized, and it is possible that dump will
end up reading the wrong data. In case that's not clear enough:
So anybody who depends on "dump" getting backups right is already
playing russian rulette with their backups. It's not at all
guaranteed to get the right results - you may end up having stale
data in the buffer cache that ends up being "backed up".
For now, there is really no easy way to fix dump for 2.4. If you're
using it, this might be a good time to go looking for a different tool.
References
20. http://lwn.net/2001/0503/a/metadata-corruption.php3
21. http://lwn.net/2001/0503/a/lt-metadata.php3
22. http://lwn.net/2001/0503/a/lt-dump.php3
===========================================================================
Mike
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