On 2016-01-29 15:27, Henk Slager wrote:
On Fri, Jan 29, 2016 at 1:14 PM, Austin S. Hemmelgarn
<ahferroin7@xxxxxxxxx> wrote:
On 2016-01-28 18:01, Chris Murphy wrote:
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?
Well, depending on how the kernel talks to the device, there are ways around
this, but most of them are slow (like waiting for the write cache to drain).
Just like SCT ERC, most drives marketed for 'desktop' usage don't actually
support FUA, but they report this fact correctly, so the kernel can often
work around it. Most of the older drives that have issues actually report
that they support it, but just treat it like a no-op. Last I checked,
Seagate's 'NAS' drives and whatever they've re-branded their other
enterprise line as, as well as WD's 'Red' drives support both SCT ERC and
FUA, but I don't know about any other brands (most of the Hitachi, Toshiba,
and Samsung drives I've seen do not support FUA). This is in-fact part of
the reason I'm saving up to get good NAS rated drives for my home server,
because those almost always support both SCT ERC and FUA.
[ 0.895207] sd 2:0:0:0: [sdc] Write cache: enabled, read cache:
enabled, doesn't support DPO or FUA
SCT ERC is supported though.
This is a 4TB (64MB buffer size) WD40EFRX-68WT0N0 FirmWare 82.00A82
and sold as 'NAS' drive.
That is at the same time troubling and not all that surprising (
SSD's don't implement it so why should we?' I hate marketing
idiocy...). I was apparently misinformed about WD's disks (although
given the apparent insanity of the firmware on some of their drives,
that really doesn't surprise me either).
How long do you think data will stay dirty in the drives writebuffer
(average/min/max)?
That depends on a huge number of factors, and I don't really have a good
answer. The 1TB 7200RPM single platter Seagate drives I'm using right
now (which have a 64MB cache) take less than 0.1 second for streaming
writes, and less than 0.5 on average for scattered writes, so it's not
too bad most of the time, but it's still a performance hit, and I do get
marginally better performance by turning off the on-disk write-cache
(I've got a very atypical workload though, so YMMV).
Another thing I noticed, is that with a Seagate 8TB SMR drive (no
FUA), the drive might be doing internal (re)writes between zones a
considerable time after OS level 'sync' has finished (I think, you can
also hear the head movements although no I/O reported on OS level /
SATA level). I think it is then not just committing its dirty parts of
the 128MB buffer, that should not take so long. Since then, I am not
so sure how fast I can shutdown+switchoff the system+drive after e.g.
btrfs receive has finished. But maybe the rewriting can be interrupted
and restarted without data corruption, I hope it can, I am just
guessing.
This really doesn't surprise me, and is a large part of why I will be
avoiding SMR drives for a long as possible. The very design means that
unless you have a battery backed write-cache, you've got serious
potential to lose data due to unclean shutdowns. One which is properly
designed should have no issues with this, but proper design of anything
these days is becoming the exception, not the rule.
--
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
