Sun Storage 7000 Unified Storage System: LUN uses more Space than the Created Size

Asset ID:	1-72-1914108.1
Update Date:	2018-01-05
Keywords:

Solution Type Problem Resolution Sure

Solution 1914108.1 : Sun Storage 7000 Unified Storage System: LUN uses more Space than the Created Size

Applies to:

Sun Storage 7110 Unified Storage System - Version All Versions to All Versions [Release All Releases]
Sun Storage 7210 Unified Storage System - Version All Versions to All Versions [Release All Releases]
Sun Storage 7310 Unified Storage System - Version All Versions to All Versions [Release All Releases]
Sun Storage 7410 Unified Storage System - Version All Versions to All Versions [Release All Releases]
Sun ZFS Storage 7320 - Version All Versions to All Versions [Release All Releases]
7000 Appliance OS (Fishworks)

Symptoms

A LUN created on the storage is using more space than assigned.

For example:

Created a 4TB LUN, but the used size is 5.12TB.

shares (pool-0) default/lun> ls
                  volblocksize = 4K
                       volsize = 4T
                    space_data = 5.12T
                   space_total = 5.12T

Cause

This is related to the pool configuration type and the volume block size used.

On a Raidz2 pool configuration with 4k block size and below uses much more disk spaces.

Raidz2 pool configuration:

Block size      Used    Volsize
1k              344M    100M
2k              172M    100M
4k              128M    100M
8k              106M    100M
16k             105M    100M
32k             100M    100M
64k             100M    100M
128k 100M    100M

Solution

You can choose a larger block size for the Raidz2 pool configuration. Or go with a Mirror pool configuration.

As shown below, the Mirror pool configuration does not use much more space even on smaller block size.

Mirror pool configuration:

Block size      Used    Volsize
1k              107M    100M
2k              102M    100M
4k              100M    100M
8k              100M    100M
16k             100M    100M
32k             100M    100M
64k             100M    100M
128k 100M    100M

According to the following bug:

Bug 18674320 - zvol lun used size have grown well over the volsize (Closed as a duplicate of Bug 15332291)

> The 'used' size is more than 'volsize' for a LUN is a normal behavior on RAIDZ configurations
> as it is the same case in upstream Solaris also.

See also Bug 15332291 - record size needs to affect zvol reservation size on RAID-Z (Fixed in 2013.1.6.0)

> When a zvol is created, the user specifies its size and ZFS creates a reservation for that size
> to ensure that the zvol can satisfy requests from consumers. On a pool backed by RAID-Z vdevs,
> small block sizes can lead to situations where sectors are frequently skipped. This can lead to
> logical data consuming more physical space than anticipated.
>
> Consider a 100MB zvol with a 2K block size on a pool backed by RAID-Z in a 4+1 configuration.
> Each block uses 4 sectors + 1 parity sector + 1 skipped sector (since each allocation must be a
> multiple of the minimum allocatable size -- in this case 2 sectors).
> Therefore ZFS needs to reserve not 100MB (125MB with parity), but 125MB (150MB with parity).
> (Plus a little more for meta data)

References

<BUG:18674320> - ZVOL LUN USED SIZE HAVE GROWN WELL OVER THE VOLSIZE
<BUG:15652925> - SUNBT6965086 ADD SUPPORT FOR SCSI UNMAP FOR ZVOL BACKED LOGICAL UNITS
<NOTE:1213714.1> - Sun ZFS Storage Appliance: Performance clues and considerations

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