which file system for sd card?
chris at csamuel.org
Mon Jan 26 00:20:39 CET 2009
On Mon, 26 Jan 2009 3:56:26 am Pietro "m0nt0" Montorfano wrote:
> My point is: journal could avoid errors and prevent some corruption, ok
> nothing to say about that.
Journals are really to allow the filesystem to present a consistent view after
a crash or failure and to try and avoid lengthy fsck's and the guesswork that
is entailed in them.
So for ext3 the default behaviour is to only flush the journalled metadata of
files after the file data has hit disk (hence why its' called "ordered" mode).
Other filesystems make different guarantees about journals and file data (and
so will ext3 if you mount with journal=writeback).
Regarding journal corruption - yes it can be an issue which is why ext4 (no
longer marked experimental in 2.6.28) checksums the journal.
Filesystems like btrfs (merged as very experimental for 2.6.29) take a
different approach, they are copy-on-write and so write new data blocks out to
different areas of the filesystem and then update the metadata to replace the
old with the new so you (should) have atomic updates and hence avoid the need
for journalling. Btrfs itself also checksums the data on disk so it will know
if data is corrupted and (if you have two partitions in a RAID1 arrangement)
will attempt to recover the data from the other partition (and yes, that does
Chris Samuel : http://www.csamuel.org/ : Melbourne, VIC
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