On Thu, Jan 28, 2016 at 1:44 PM, Austin S. Hemmelgarn <ahferroin7@xxxxxxxxx> wrote: >> Interesting, I figured a umount should include telling the drive to >> flush the write cache; but maybe not, if the drive or connection (i.e. >> USB enclosure) doesn't support FUA? > > It's supposed to send an FUA, but depending on the hardware, this may either > disappear on the way to the disk, or more likely just be a no-op. A lot of > cheap older HDD's just ignore it, and I've seen a lot of USB enclosures that > just eat the command and don't pass anything to the disk, so sometimes you > have to get creative to actually flush the cache. It's worth noting that > most such disks are not safe to use BTRFS on anyway though, because FUA is > part of what's used to force write barriers. Err. Really? [ 0.833452] scsi 0:0:0:0: Direct-Access ATA Samsung SSD 840 DB6Q PQ: 0 ANSI: 5 [ 0.835810] ata3.00: ACPI cmd ef/10:03:00:00:00:a0 (SET FEATURES) filtered out [ 0.835827] ata3.00: configured for UDMA/100 [ 0.838010] usb 1-1: new high-speed USB device number 2 using ehci-pci [ 0.839785] sd 0:0:0:0: Attached scsi generic sg0 type 0 [ 0.839810] sd 0:0:0:0: [sda] 488397168 512-byte logical blocks: (250 GB/233 GiB) [ 0.840381] sd 0:0:0:0: [sda] Write Protect is off [ 0.840393] sd 0:0:0:0: [sda] Mode Sense: 00 3a 00 00 [ 0.840634] sd 0:0:0:0: [sda] Write cache: enabled, read cache: enabled, doesn't support DPO or FUA This is not a cheap or old HDD. It's not in an enclosure. I get the same message for a new Toshiba 1TiB drive I just stuck in a new Intel NUC. So now what? >> If I don't, my drives make a loud clank, and the smart attribute 192 >> Power-off Retract Count, goes up by one. This never happens on a >> normal power off. So some message is being sent to the drive at >> restart/poweroff that's different than just pulling the drive, even if >> that message isn't the same thing as whatever hdparm -Y sends. >> > I'm not saying it's a good idea to not tell the drive to spin down, just > that it won't damage most modern drives as long as they're kept level while > they spin down and you don't do it all the time. Gotcha. > > Almost every modern hard disk uses a voice-coil actuator for the heads which > gets balanced such that having no power to the coil causes the forces from > the spinning disks to park the heads, so pulling power will (more than 99.9% > of the time) not cause a head cash like a lot of older servo based drives as > long as you keep the drive level. The clank you hear is the end of the head > armature opposite the heads hitting the mechanical stop that's present to > prevent them from completely decoupling from the disk. This gets accounted > in SMART attributes because over extremely long times (usually tens > thousands of cycles), this will eventually wear out that mechanical stop, > and things will stop working, so it technically is a failure condition, but > you're almost certain to hit some other failure condition before this > becomes an issue. OK. > > The interesting thing is that some drives actually _rely_ on this behavior > to park the heads (I've seen a lot of Seagate desktop drives that appear to > do this, although they use a rubber stopper instead of metal or plastic, so > it tends to last longer). Cute. -- Chris Murphy -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
