ReFS VS Dell Compellent

[Ctek]

Hi,

We have been using ReFS with a lot of trial and errors over the last 6 months, but we lately found out that the space savings in the OS of a Backup Repository were not showing on the Thin-Provisioning Dell Compellent SC-Series SANs. Turns out that Dell Compellent does not support TRIM/Unmap with ReFS LUNs. While we do have other advantages with ReFS, it appears that the space savings on thin Dell SC SANs is not something to factor in.

https://www.google.ca/url?sa=t&rct=j&q= ... WEQn1_-sGR

[s_t]

Space saving through thin provisioning and the benefits of ReFS are not the same thing:

TRIM/UNMAP in combination with thin provisioned volumes on storages that support this feature allows shrinking volumes with space that was previously allocated by blocks with files that are now deleted (if the blocks are not already used again)

ReFS has some space saving advantages as it is possible for synthetic jobs to re-use/link blocks that are the same in retention points so that they do not need the capacity for both retention points (not writing identical blocks is better then deduplicating identical blocks afterwards)

You should see some significant space saving also with ReFS on DELL Compellent Storage, if you have Jobs that do synthetic full backups

But there is another reason to not use ReFS on Storage Systems (independent of Compellent or other storages):
ReFS is officially not supported on SAN:
"ReFS is not supported with hardware virtualized storage such as SANs" (https://docs.microsoft.com/en-us/window ... s-overview)

[Andreas Neufert]

ReFS stability depends as well on the SAN storage system follow the flush command 100% which is not the case for all external storages. I guess it is hard to create a HCL for this and so the statement form Microsoft is a more general one.

We had customers that opened an advisory case and became positive feedback from Microsoft for their storage situation.

[nmdange]

Dell Compellent as backup storage? Seems a bit expensive to use for backups!

[Andreas Neufert]

depending on the use case scenario and PB of data those primary storage systems are used as backup targets.

[CKESSLER]

I add a comment to this topic, since I just have an issue with ReFS and our Dell Compellent.

We use our Compellent as backup repository and therefore provided a 30TB LUN.
Our Veeam server is a Server 2016 and the volume is formatted with ReFS.
Over the last year I was totally satisfied with the performance. Backup speed was great and the weekly synthetic full backups also took only 10minutes.

Now this week I had this issue:
Noticed that the Compellent shows a warning of only 4% free disk space left of the 30TB.
While the Windows ReFS Volume still shows about 40% free space.

Talked with Compellent Support:
Problem is, that ReFS supports no TRIM/Unmap. Therefore deleted data will not be marked again as free space.
I did not know that, even if it was declared in the MS technet.

Dell Support clearly recommended to use NTFS in this case.

So I guess that I will reconfigure everything back to NTFS which might result in lower performance and space savings.

But would be glad if anyone has better recommendations or comments

[Andreas Neufert]

So we have the situation where you rolled out the disk "thin" on the Compellent.

Based on what the support said is, that ReFS will not give non used data blocks as free to the storage, which basically means that near 100% of the volume size is used within Windows. A backup target is designed to use the given storage anyway, so you should not overprovision your storage system.

If you go to NTFS, you maybe get free space back to the storage, but if you have overprovisioned the storage, you will run into trouble anyway.

With NTFS you will loose block cloning and the daily merges or weekly synthetic full processing will take longer.

[mitchellmans2203]

Using Dell Compellent with REFS did you ever have issues with the disks going offline mid backup operations?

[lmbutault]

hello,

remember that ReFS is not really recomended by Microsoft on SAN devices, also not recommended with thin provisioning
https://docs.microsoft.com/en-us/window ... s-overview

"Please contact application and storage array vendors for support details on Fiber Channel and iSCSI SANs. For SANs, if features such as thin provisioning, TRIM/UNMAP, or Offloaded Data Transfer (ODX) are required, NTFS must be used."

regards.

[CKESSLER]

Maybe someone is interessted in my experience, 1 1/2 years after my last post in this thread:

I did NOT change back to NTFS on the Compellent. As far as I remember we extended the disk shelf that time to 100TB.

The main backup volume / LUN has currently a size of 40TB and Compellent shows a usage of 96%. This has been the status over the last year.
But on the Windows volume, I only have a usage of 75%.

A second volume (O365 Backup) has a size of 2TB, 62% usage.

If I take a look on the hard disks / RAID group there i have 17TB free and unassigned space left.

So my conclusion is that somehow Windows uses reuses the deleted data on the ReFS volume.
Really the only issue I have is a slow data write / read rate, since I only have SATA disks.

@mitchellmans2203:
I did not experience any problems with disks going offline. You mean the volume connected in Windows or the (hardware) disks in the Compellent?