Synology NAS with an iSCSI-mounted LUN formatted in ReFS on Windows
Posted by Wrong-Slide-3240@reddit | sysadmin | View on Reddit | 16 comments
I’m having an issue with a Synology NAS storage setup using a LUN mounted via iSCSI and formatted with ReFS on Windows. I use the ReFS partition for my Veeam backups.
On Windows, the disk shows 10 TB of free space. However, on the Synology NAS volume, the available space keeps decreasing and I now have only 500 GB left.
I tried running commands like Optimize-Volume, but they didn’t reclaim any space.
FearFactory2904@reddit
Silly rabbit, you can't reclaim unused space in refs.
Wrong-Slide-3240@reddit (OP)
I think the problem is the Thin provisioning on the Syno LUN
FearFactory2904@reddit
Wait, is your 'problem' that the free spaces don't match up and think it's abnormal, or that you want the difference back because you need the extra space for other luns? The server knows the space at a filesystem level where the syno is only block level aware of the contents. Over time from writing, moving, and deleting files they will 'touch' more and more block which means this is expected behavior (assuming you cant reclaim the difference with scsi unmap.) On top of that you said you are using REFS, so you can't reclaim the difference with scsi unmap. So to me, I don't really see a problem in your scenario so much as 'yep, it do be like that.' If you were not thin provisioning then the entire space would be pre allocated on the synology end and this would all be a moot point. Once the lun shows full from the SAN end it is effectively like you have a thick provisioned lun, which is fine as long as you didn't overprovision your storage.
Wrong-Slide-3240@reddit (OP)
In theory, yes, I agree, but I've already encountered this issue before under the same conditions. The volume fills up completely, drops to less than 1% available, and then goes into a safe mode, making it impossible to write to it.
FearFactory2904@reddit
That is some shit foresight in the SAN vendors part then. Most SAN have a safe mode but that is if all the disk space for the entire disk pool runs low. If you happened to make 1 thin provisioned volume and set it to use all of the space for the entire pool then filling up the volume could simultaneously meet both conditions but if you are reserving a reasonable amount of space when you create the volume then the vendor should simply allow the volume to fill without complaining. Otherwise for example, if somebody were to do the long slow initialization of a drive instead of the quick format you would end up hitting safe mode before actually getting a chance to write any files to your new filesystem because technically all the space from the block level perspective would be "full" of zeroes. In this case you may need to thick provision of the storage allows or use NTFS instead of REFS so that you have the ability to reclaim freed up blocks via scsi unmap/trim (This also assumes the array supports scsi unmap. In my opinion they should not even offer thin provisioning as an option if they don't.)
malikto44@reddit
Are there snapshots being done on the Synology side? I'm also wondering if this is thin provisioned.
Wrong-Slide-3240@reddit (OP)
Yes, there are some Snapshots but it takes only 200Gb.
And yes, it is Thin provisioning
somoa20@reddit
I have snapshots disabled on my synology LUN that Veeam is using
somoa20@reddit
Have similar setup for Veeam, what file system is on the Synology volume?
Wrong-Slide-3240@reddit (OP)
On Synology NAS
Balthxzar@reddit
Are you using a fixed size iSCSI disk? From memory, the iSCSI LUN will appear as a VHDX/VHD of whatever size is provisioned to the host OS
Wrong-Slide-3240@reddit (OP)
On the Syno it's in Thin provisionning
Balthxzar@reddit
I'm not sure if windows with ReFS is aware at that level of it actually being thin.
Wrong-Slide-3240@reddit (OP)
Yes, I think it's the thing
Balthxzar@reddit
I'd always say that unless every part interacting with the volumes is aware of the provisioning, just thick provision. It avoids issues of one service/host/etc taking over space that is already held by other volumes
Wrong-Slide-3240@reddit (OP)
On Windows