[Beowulf] Re: Large raid rebuild times

Paulo Afonso Lopes pal at di.fct.unl.pt
Fri Nov 20 04:13:12 PST 2009

(Sorry: I forgot to cc the mailing list)

> wouldnt the limiting factor be the read and write times of the drives in
regards to rebuilding an array?

I would expect the write BW of a single drive (the one being rebuilt) in
sequential access mode to be the limiting factor.

If, in a 5 drive RAID-5, a single drive sustains, say, 50 MB/s write BW
(and, say, 50MB/s read for simplicity):

  - time for reading 4 drives in parallel == time for reading 1 drive
   (e.g., 1TB at 50MB/s == 5,5 hours)

If the data recovery compute time is negligible (I expect it to be), then
the time to rebuild a 5 disk RAID5 with 1 TB disks should be better than
6h, provided that the "host" is able to sustain more than 250 MB/s for the

If you are using a disk-array (not an entry-level cheapo, but FC ones like
IBM DS4800, HP EVA 6000, EMC CX500 or better - these are old models) with
no activity other than the rebuilding, it should take less than 6h for a
5x 1TB/drive RAID5, IMO.


Paulo Afonso Lopes                        | Tel: +351- 21 294 8536
Departamento de Informática               | 294 8300 ext.10702
Faculdade de Ciências e Tecnologia        | Fax: +351- 21 294 8541
Universidade Nova de Lisboa               | e-mail: poral at fct.unl.pt
2829-516 Caparica, PORTUGAL

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