[TriLUG] MD & LVM Presentation
David W. Aquilina
david at starkindler.us
Thu Oct 13 21:36:38 EDT 2005
reetings all,
Hope everyone enjoyed the presentation. The slides are available at http://www.trilug.org/~dwa/md-lvm-presentation.
Regarding the question of what a 'dirty' array means, the following is from the md(4) manpage:
UNCLEAN SHUTDOWN
When changes are made to a RAID1, RAID4, RAID5 or RAID6 array there is
a possibility of inconsistency for short periods of time as each
update requires are least two block to be written to different
devices, and these writes probably wont happen at exactly the same
time. Thus if a system with one of these arrays is shutdown in the
middle of a write operation (e.g. due to power failure), the array may
not be consistent.
To handle this situation, the md driver marks an array as "dirty"
before writing any data to it, and marks it as "clean" when the array
is being disabled, e.g. at shutdown. If the md driver finds an array
to be dirty at startup, it proceeds to correct any possibly inconsis-
tency. For RAID1, this involves copying the contents of the first
drive onto all other drives. For RAID4, RAID5 and RAID6 this involves
recalculating the parity for each stripe and making sure that the par-
ity block has the correct data. This process, known as "resynchronis-
ing" or "resync" is performed in the background. The array can still
be used, though possibly with reduced performance.
http://en.wikipedia.org/wiki/Redundant_array_of_independent_disks was noted as having information about parity in the #trilug IRC channel, however I've not verified it.
Thanks for your attention, and feel free to send any questions my way, or to the list.
best,
--
David W. Aquilina
david at starkindler.us
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