OMV 5 + SnapRAID and very slow sync speed

  • Hi all,

    Short-time lurker, first-time poster.

    I have to say that I've found the forum extremely helpful for my journey into DIY NAS solutions. Unfortunately, I have reached a point that I cannot overcome. I've searched the forum and the almighty Google but have not been able to find an answer to my problem. So here it goes...

    I have a Dell T310 (X3430, 24 GB RAM, 3 x 4TB Seagate SAS drives connected to an SPL-3 HBA).

    1.) I have installed OMV 5.3.4-1 and am using unionfs with SnapRAID.

    2.) I went through the process of adding the two disks to a pool in unionfs and then added those disks as data disks in SnapRAID.

    3.) I moved my data to the pool, which is approximately 4.7TB of data.

    4.) After I was done migrating the data, I added a parity disk in SnapRAID.

    Now, here's the problem. I selected the "Sync" option in SnapRAID to start the initial sync process and I am seeing speeds of 53 MB/s. No fluctuations, just 53 MB/s. The fact that it isn't fluctuating at all, makes me think there's some kind of bottleneck, however, I can't determine where. The drives seem to transfer at around 112 MB/s over my network without issue.

    Any ideas?

  • I'm not entirely sure what determines snapraid sync speeds, but here's my experience: the more drives you have in the pool, the faster it goes. But also more drives mean that more data is synced. For example if you have 3 drives and add a 100MB file, it will sync 300 MB. I'm up to 9 drives in my pool and the sync speeds are often >1000 MB/s and I use WD reds and whites so I know the drives aren't that fast. If I add a 100MB file, it will sync 900MB.

    Now, it's been years since I did an initial sync. That might just be a slower process and you need to wait it out. Also, are you using pre-hash? I think your 53MB/s is probably just the hashing step. Then when that completes and it does the sync step, it will go faster. The pre-hash is purposely slower.

    From the manual:

    -h, --pre-hash
    In "sync" runs a preliminary hashing phase of all the new data to have an additional verification before the parity computation. Usually in "sync" no preliminary hashing is done, and the new data is hashed just before the parity computation when it's read for the first time. Unfortunately, this process happens when the system is under heavy load, with all disks spinning and with a busy CPU. This is an extreme condition for the machine, and if it has a latent hardware problem, it's possible to encounter silent errors what cannot be detected because the data is not yet hashed. To avoid this risk, you can enable the "pre-hash" mode and have all the data read two times to ensure its integrity. This option also verifies the files moved inside the array, to ensure that the move operation went successfully, and in case to block the sync and to allow to run a fix operation. This option can be used only with "sync".

    I always use sync with pre-hash.

  • I was just using the “sync” option within the SnapRAID tools menu.

    The interesting thing is that I had an identical setup on unRAID and would get 180 MB/s during the initial sync. As you said, maybe it’s the prehash or even just the fact that it’s an initial sync.

    I am now attempting to run SnapRAID brought Windows CLI to test sync speeds.

    This could also just be a dying drive, I suppose!

  • I have two 12TB Baracuda Pro HDDs I bought as "new, refurbished" because I have a similarly refurbished 10TB HGST running for years without problems. I set up a Snapraid with both of them and copied 10TB of data from my old RAID to the data disc in 20 hours. Now, the initial sync is running and is shown in the GUI as "28MB/s". So, I guess, it has nothing to do with bad disks but might be related to the initial sync.

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