On 10/19/16 19:15, Austin S. Hemmelgarn wrote:
On 2016-10-18 17:36, Anand Jain wrote:
I would like to monitor my btrfs-filesystem for missing drives.
This is actually correct behavior, the filesystem reports that it
should
have 6 devices, which is how it knows a device is missing.
Missing - means missing at the time of mount. So how are you planning
to monitor a disk which is failed while in production ?
No, in `btrfs fi show` it means that it can't find the device.
'btrfs fi show' is miss-leading as compared to 'btrfs fi show -m'
-m tells btrfs-kernel perspective of the devices, as of now
there is no code in the kernel which changes the device status
while its mounted (expect for readonly, which is irrelevant in
raid1 with 1 disk failed).
Actually, that's exactly how I would expect each of them to behave. We
need some way to get both the state the kernel thinks the FS is in, and
the state it's actually in (according to the tools, not the kernel), and
'-m' reporting kernel state while no '-m' reports actual state is
exactly what I would expect in this case.
That leads also to another way I hadn't thought of to monitor a
filesystem. The output of 'fi show' with and without '-m' should match
if the filesystem was healthy when mounted and is still healthy, if they
don't, then something is wrong.
1. Filesystem flags. These will change when the filesystem goes
degraded,
Which flag is in question here. ?
I should clarify here, I mean the mount options, I'm just used to the
monit terminology (which was not well picked in this case). The big one
to watch is the read-only flag, as BTRFS will force a filesystem
read-only (which updates the mount options). Any change to the mount
options though without manual intervention is generally a sign that
_something_ is wrong.
btrfs-progs shouldn't add its own intelligence in determining the
device state, it should be a transparent tool to report status from
the btrfs-kernel. So I opposed to the patches such as
commit 206efb60cbe3049e0d44c6da3c1909aeee18f813
btrfs-progs: Add missing devices check for mounted btrfs.
There are many ways a device can fail/recover in the SAN environment,
these device state managing intelligence should be at one place and
in the kernel. The volume manager part of the code in the kernel
is incomplete.
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