Re: [patch] ext2/3: document conditions when reliable operation ispossible

From: david
Date: Tue Aug 25 2009 - 11:36:00 EST

On Tue, 25 Aug 2009, Pavel Machek wrote:


If your concern is that with Linux MD, you could potentially lose an
entire stripe in RAID 5 mode, then you should say that explicitly; but
again, this isn't a filesystem specific cliam; it's true for all
filesystems. I don't know of any file system that can survive having
a RAID stripe-shaped-hole blown into the middle of it due to a power

Again, ext2 handles that in a way user expects it.

At least I was teached "ext2 needs fsck after powerfail; ext3 can
handle powerfails just ok".

So, would you be happy if ext3 fsck was always run on reboot (at least
for flash devices)?

For flash devices, MD Raid 5 and anything else that needs it; yes that
would make me happy ;-).

the thing is that fsck would not fix the problem.

it may (if the data lost was metadata) detect the problem and tell you how many files you have lost, but if the data lost was all in a data file you would not detect it with a fsck

the only way you would detect the missing data is to read all the files on the filesystem and detect that the data you are reading is wrong.

but how can you tell if the data you are reading is wrong?

on a flash drive, your read can return garbage, but how do you know that garbage isn't the contents of the file?

on a degraded raid5 array you have no way to test data integrity, so when the missing drive is replaced, the rebuild algorithm will calculate the appropriate data to make the parity calculations work out and write garbage to that drive.

David Lang
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