Linda Walsh wrote: > > Michael Tokarev wrote: >> Unfortunately an UPS does not *really* help here. Because unless >> it has control program which properly shuts system down on the loss >> of input power, and the battery really has the capacity to power the >> system while it's shutting down (anyone tested this? > ---- > Yes. I must say, I am not connected or paid by APC. > >> With new UPS? >> and after an year of use, when the battery is not new?), -- unless >> the UPS actually has the capacity to shutdown system, it will cut >> the power at an unexpected time, while the disk(s) still has dirty >> caches... > -------- > If you have a "SmartUPS" by "APC", their is a freeware demon that monitors [...] Good stuff. I knew at least SOME UPSes are good... ;) Too bad I rarely see such stuff in use by regular home users... [] >> Note also that with linux software raid barriers are NOT supported. > ------ > Are you sure about this? When my system boots, I used to have > 3 new IDE's, and one older one. XFS checked each drive for barriers > and turned off barriers for a disk that didn't support it. ... or > are you referring specifically to linux-raid setups? I'm referring especially to linux-raid setups (software raid). md devices don't support barriers, because of a very simple reasons: once more than one disk drive is involved, md layer can't guarantee ordering ACROSS drives too. The problem is that in case of power loss during writes, when an array needs recovery/resync (at least the parts which were being written, if bitmaps are in use), md layer will choose arbitrary drive as a "master" and will copy data to another drive (speaking of simplest case of 2-drive raid1 array). But the thing is that one drive may have two last barriers written (I mean the data that was "assotiated" with the barriers), and another neither of the two - in two different places. And hence we may see quite.. some inconsistency here. This is regardless of whether underlying component devices supports barriers or not. > Would it be possible on boot to have xfs probe the Raid array, > physically, to see if barriers are really supported (or not), and disable > them if they are not (and optionally disabling write caching, but that's > a major performance hit in my experience. Xfs already probes the devices as you describe, exactly the same way as you've seen with your ide disks, and disables barriers. The question and confusing was about what happens when the barriers are disabled (provided, again, that we don't rely on UPS and other external things). As far as I understand, when barriers are working properly, xfs should be safe wrt power losses (still a bit unsure about this). Now, when barriers are turned off (for whatever reason), is it still as safe? I don't know. Does it use regular cache flushes in place of barriers in that case (which ARE supported by md layer)? Generally, it has been said numerous times that XFS is not "powercut-friendly", and it has to be used when everything is stable, including power. Hence I'm afraid to deploy it where I know the power is not stable (we've about 70 such places here, with servers in each, where they don't always replace UPS batteries in time - ext3fs never crashed so far, while ext2 did). Thanks. /mjt - 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
