> -----Original Message----- > From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid- > owner@xxxxxxxxxxxxxxx] On Behalf Of Mario 'BitKoenig' Holbe > Sent: Tuesday, October 21, 2008 9:12 AM > To: linux-raid@xxxxxxxxxxxxxxx > Subject: Re: Proactive Drive Replacement > > David Lethe <david@xxxxxxxxxxxx> wrote: > > S.M.A.R.T. does not, has not, will not, ever ... identify bad blocks. > > Well, as you state yourself later, S.M.A.R.T. defines self-tests which > are able to identify bad blocks. Though, they have to be triggered. > > > Both families of disks provide for some self-test commands, but these > > commands do not scan the > > entire surface of the disk > > This is not true. The long self-test scans the entire surface of the > disk at least for ATA devices, I don't know if it does that for SCSI > devices too. > ATA does also know about selective self-tests which are able to scan > defineable surface areas - which is, at first, quite nice to identify > more than one bad sector, and which is, at second, quite nice on bigger > devices as well... my ST31500341AS take about 4.5 hours for a long > self-test. > > > new bad block. They report if you have a bad block if one is found > in > > the extremely small sample > > of I/O it ran. > > And, at least ATA devices report the LBA_of_first_error in the self- > test > log, so you can identify the first bad sector. > > > regards > Mario > -- > Singing is the lowest form of communication. > -- Homer J. Simpson > > -- > To unsubscribe from this list: send the line "unsubscribe linux-raid" > in > the body of a message to majordomo@xxxxxxxxxxxxxxx > More majordomo info at http://vger.kernel.org/majordomo-info.html The SCSI-family of self-test commands terminate after the first media error. This makes perfect sense as if the disk fails, you ordinarily want to know that immediately, rather than have the disk continue scanning. As such, self-test gives you the first bad block and that is it. As for SATA/ATA self-tests, then all logs are limited to 512 bytes. If you run the right self-test, then You will get a PARTIAL list of bad blocks. Specifically, you get 24 bytes which tell you the starting bad Block. You do not even get a range of bad blocks. You just know that block X is bad. It doesn't tell you if block X+1 is bad. If block X+2 is bad, it will tell you that, because it chews up another log entry. There is room for 20 entries. Not all disks support this type of self-test either. The ANSI spec says this is optional, and it is a relatively recent introduction. So, at best, if you disk supports it, you can run self-tests that will take half a day and give you a partial list of bad blocks, between ranges of LBA numbers you want to scan. This is correctly called the "SMART selective self-test routine". By the way, this is an OFF-LINE scan. So bottom line, Mario is correct in that there is a way to get a PARTIAL list of bad blocks, if you have a disk that supports this command, and you're willing to run an off-line scan (not practical or a parity-protected RAID environment). As original poster wanted to just use SMART to factor in known bad blocks on a rebuild, then you can see that there Is no viable option unless you already have a full list of known bad blocks. You have to find bad blocks as you just read from them as part of the rebuild for these types of disks). It is possible that some vendor has implemented a SATA ON-LINE bad block scanning mechanism that reports results and doesn't kill I/O performance. It would have to give full list of bad blocks, or at least startingblock + range. That would be wonderful as you could just read the list on regular interval and rebuild stripes as necessary. You'd have Self-healing parity. It still wouldn't protect against a drive failure, but it would insure that you wouldn't have any lost chunks due to a unreadable block on one of the surviving disks in a RAID set. David -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
