VDO Killed My Server
Folks
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.
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…), 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.
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.
– The documentation talks about ‘tuning’, but if this resource is one of concern, please don’t bury it in the footnotes to the appendix.
– Using VDO on top of LVM seems to be the logical way to use deduplication for a large file set, yet the use cases don’t seem to cover this (unless I misread them)
I have reverted back reluctantly to using ZFS for this function.
Have others had issues with VDO?
David Kurn Linux amateur
One thought on - VDO Killed My Server
Interesting observation! I’m thinking about trying VDO too.
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.
Did these failures to create a file occur only on the file system on VDO
or also on other file system?
How about free memory? What did `free -m` say?
Definitely.
I agree. Tuning should only affect performance, never normal functionality.
—
Yan Li