On Sun, Sep 14, 2014 at 05:15:08AM +0200, Piotr Pawłow wrote: > On 12.09.2014 12:47, Hugo Mills wrote: > >I've done this before, by accident (pulled the wrong drive, reinserted > >it). You can fix it by running a scrub on the device (btrfs scrub > >start /dev/ice, I think). > > I'd like to remind everyone that btrfs has weak checksums. It may be good > for correcting an occasional error, but I wouldn't trust it to correct > larger amounts of data.
Checksums are done for each 4k block, so the increase in probability of a false negative is purely to do with the sher volume of data. "Weak" checksums like the CRC32 that btrfs currently uses are indeed poor for detecting malicious targeted attacks on the data, but for random failures, such as a disk block being unreadable and returning zeroes or having bit errors, the odds of identifying the failure are still excellent. > Additionally, nocow files are not checksummed. They will not be corrected > and may return good data or random garbage, depending on which mirror is > accessed. Yes, this is a trade-off that you have to make for your own use-case and happiness. For some things (like a browser cache), I'd be happy with losing the checksums. For others (e.g. mail), I wouldn't be. Hugo. > Below is a test I did some time ago, demonstrating the problem with nocow > files: > > #!/bin/sh > MOUNT_DIR=mnt > DISK1=d1 > DISK2=d2 > SIZE=2G > # create raid1 FS > mkdir $MOUNT_DIR > truncate --size $SIZE $DISK1 > truncate --size $SIZE $DISK2 > L1=$(losetup --show -f $DISK1) > L2=$(losetup --show -f $DISK2) > mkfs.btrfs -d raid1 -m raid1 $L1 $L2 > mount $L1 $MOUNT_DIR > # enable NOCOW > chattr +C $MOUNT_DIR > umount $MOUNT_DIR > # fail the second drive > losetup -d $L2 > mount $L1 $MOUNT_DIR -odegraded > # file must be large enough to not get embedded inside metadata > perl -e 'print "Test OK.\n"x4096' >$MOUNT_DIR/testfile > umount $MOUNT_DIR > # reattach the second drive > L2=$(losetup --show -f $DISK2) > mount $L1 $MOUNT_DIR > # let's see what we get - correct data or garbage? > cat $MOUNT_DIR/testfile > # clean up > umount $MOUNT_DIR > losetup -d $L1 > losetup -d $L2 > rm $DISK1 $DISK2 > rmdir $MOUNT_DIR -- === Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk === PGP key: 65E74AC0 from wwwkeys.eu.pgp.net or http://www.carfax.org.uk --- Hey, Virtual Memory! Now I can have a *really big* ramdisk! ---
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