Re: [patch] document flash/RAID dangers

From: david
Date: Tue Aug 25 2009 - 19:58:15 EST

On Wed, 26 Aug 2009, Pavel Machek wrote:

There are storage devices that high highly undesirable properties
when they are disconnected or suffer power failures while writes are
in progress; such devices include flash devices and MD RAID 4/5/6

change this to say 'degraded MD RAID 4/5/6 arrays'

also find out if DM RAID 4/5/6 arrays suffer the same problem (I strongly suspect that they do)

then you need to add a note that if the array becomes degraded before a scrub cycle happens previously hidden damage (that would have been repaired by the scrub) can surface.

These devices have the property of potentially corrupting blocks being written at the time of the power failure,

this is true of all devices

and worse yet, amplifying the region where blocks are corrupted such that additional sectors are also damaged during the power failure.

re-word this something like

In addition to the standard risk of corrupting the blocks being written at the time of the power failure, additonal blocks (in the same flash eraseblock or raid stripe) may also be corrupted.

Users who use such storage devices are well advised take
countermeasures, such as the use of Uninterruptible Power Supplies,
and making sure the flash device is not hot-unplugged while the device
is being used. Regular backups when using these devices is also a
Very Good Idea.

Otherwise, file systems placed on these devices can suffer silent data
and file system corruption. An forced use of fsck may detect metadata
corruption resulting in file system corruption, but will not suffice
to detect data corruption.

David Lang
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