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.