[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



More information about the TriLUG mailing list