On Thu, Aug 20, 2009 at 10:42 AM, Ric Wheeler<rwheeler@xxxxxxxxxx> wrote: > On 08/20/2009 10:38 AM, Mark Lord wrote: >> >> Ric Wheeler wrote: >>> >>> Note that returning consistent data is critical for devices that are >>> used in a RAID group since you will need each RAID block that is used >>> to compute the parity to continue to return the same data until you >>> overwrite it with new data :-) >>> >>> If we have a device that does not support this (or is misconfigured >>> not to do this), we should not use those devices in an MD group & do >>> discard against it... >> >> .. >> >> Well, that's a bit drastic. But the RAID software should at least >> not issue TRIM commands in ignorance of such. > > If the storage can return different data in a sequence of READ requests of > the same sector (with no writes), there is nothing RAID could do. It would > see total garbage... > >> Would it still be okay to do the TRIMs when the entire parity stripe >> (across all members) is being discarded? (As opposed to just partial >> data there being dropped) > > This should be safe if the MD bitmaps would prevent us from trying to > READ/regenerate parity for that stripe... > > ric The harder thing for mdraid is putting a stripe back in service. If even a single sector is written to a "discarded" stripe, the entire stripe has to be written with determinate data that has the right parity. ie. Only full stripes can be discarded and only full-stripes can be put back in service. Greg -- Greg Freemyer Head of EDD Tape Extraction and Processing team Litigation Triage Solutions Specialist http://www.linkedin.com/in/gregfreemyer Preservation and Forensic processing of Exchange Repositories White Paper - <http://www.norcrossgroup.com/forms/whitepapers/tng_whitepaper_fpe.html> The Norcross Group The Intersection of Evidence & Technology http://www.norcrossgroup.com -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
