[mythtv-users] 2 TB Hard Drive Recommendations

Simon Hobson linux at thehobsons.co.uk
Mon Dec 6 23:20:51 UTC 2010

Richard Morton wrote:
>Just want to point out RAID5 is not a good solution for partitions
>where lots of small modifications are made... such as for DBs in which
>case use RAID 1 or RAID 1+0 (RAID 10).
>  - on a RAID 5 array, it has to read all the drives not being
>modified, the checksum is calculated and the data is written and the
>parity is written two the two drives that need to be changed...

Actually, what I believe most controllers do is :
Read the block being overwritten and the parity block.
XOR the old parity block with the old data block, then with the new 
data block to get a new parity block.
Write the new data block and parity block.

For larger arrays this is quicker, but for minimal arrays (eg 3 
drives) it may be slower. Of course, for larger sequential writes, 
the parity block may already be in cache; and for read-modify-write 
cycles, the old data may still be in cache.

The big downside to this is that if the parity block is corrupt, the 
corruption is carried forward. You would never know about it until 
you have a drive failure and it is used to rebuild a drive - I've 
seen this called the RAID5 Hole.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
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