Interesting observation! I'm thinking about trying VDO too.
On 09/03/2018 11:40 AM, david wrote:
I was impressed with the description of VDO (Virtual Device Optimizer?)
in the RedHat documentaion, so much that I tried to use it. The
tutorials led me to a few commands. I built a VDO device on top of two
USB disks which I made into a Logical Volume, and I was ready to go.
USB connections are notorious flimsy. They are prone to randomly
dropping ops and silent intermittent connection breaks, and are known to
cause a lot of hard-to-debug problems when being used with more complex
filesystems, such as ZFS and btrfs. Of course we shouldn't blame USB for
every problem, but I wouldn't be surprised if USB is playing naughty here.
In my test case, I had a file set of about 600 GB. There was 5 TB of
space between the two disk LVMs. So, I thought, let's see if I can
activate deduplication and compression, and see if VDO can take two, or
three, or four identical copies of that file set, at different points in
the file system tree.
Needless to say, all worked well with the first set. It took 24 hours
to copy. The second set took another 24 hours, and all seemed well. As
I was copying the third set, I started to observe some problems. The
computer was serving other functions (internal DHCPD, DNS, internal
HTTPD), and these started to fail. There were no obvious alerts or
warnings from VDO, but the other functions of the system started to
die. The diagnostics from JOURNALCTL were vague (failure to create a
file...),
Did these failures to create a file occur only on the file system on VDO
or also on other file system?
but when I want looking with 'df', all the file systems seemed
to have enough room for everything. Even the 'top' program showed
available space in the pools it revealed.
How about free memory? What did `free -m` say?
After hours of my internal clients complaining, I finally removed the
'mount' in /etc/fstab that loaded the VDO system, killed the file
copies, and rebooted. The system then resumed normal healthy functions,
but without the VDO files.
It my mind, there are a few points:
- If VDO is competing for a finite resource (Memory?), it probably
should start posting warnings, and eventually rejecting new files when
the pool is nearly full. Or maybe, use a pool other than what the other
services use so as to minimize the impact on them.
Definitely.
- The documentation talks about 'tuning', but if this resource is one of
concern, please don't bury it in the footnotes to the appendix.
I agree. Tuning should only affect performance, never normal functionality.
--
Yan Li
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