SSD Drives

So I had two sets of SSDs in my VMware setup, one on the local mac mini:

[root@macm:~] esxcli storage core device list -d t10.ATA_APPLE_SSD_SM256E
t10.ATA_APPLE_SSD_SM256E
   Display Name: Local ATA Disk (t10.ATA_APPLE_SSD_SM256E)
   Has Settable Display Name: true
   Size: 239372
   Device Type: Direct-Access
   Multipath Plugin: NMP
   Devfs Path: /vmfs/devices/disks/t10.ATA_APPLE_SSD_SM256E
   Vendor: ATA
   Model: APPLE SSD SM256E
   Revision: 2A0Q
   SCSI Level: 5
   Is Pseudo: false
   Status: on
   Is RDM Capable: false
   Is Local: true
   Is Removable: false
   Is SSD: true
   Is VVOL PE: false
   Is Offline: false
   Is Perennially Reserved: false
   Queue Full Sample Size: 0
   Queue Full Threshold: 0
   Thin Provisioning Status: yes
   Attached Filters:
   VAAI Status: unknown
   Other UIDs: vml.0100000000533141414e594e463330323932342020202020204150504c4520
   Is Shared Clusterwide: false
   Is Local SAS Device: false
   Is SAS: false
   Is USB: false
   Is Boot USB Device: false
   Is Boot Device: true
   Device Max Queue Depth: 31
   No of outstanding IOs with competing worlds: 32
   Drive Type: unknown
   RAID Level: unknown
   Number of Physical Drives: unknown
   Protection Enabled: false
   PI Activated: false
   PI Type: 0
   PI Protection Mask: NO PROTECTION
   Supported Guard Types: NO GUARD SUPPORT
   DIX Enabled: false
   DIX Guard Type: NO GUARD SUPPORT
   Emulated DIX/DIF Enabled: false

And a couple on my OmniOS ZFS storage:

<> sg_vpd -p ai /dev/rdsk/c2t4d0
ATA information VPD page:
  SAT Vendor identification: ATA     
  SAT Product identification: Samsung SSD 840 
  SAT Product revision level: BB6Q
  Device signature indicates PATA transport
  ATA command IDENTIFY DEVICE response summary:
    model: Samsung SSD 840 EVO 250GB               
    serial number: ftrt     
    firmware revision: EXT0BB6Q

SSD Performance Degradation

Over a period of 4 years, the SSDs became slow and slower. I noticed that backups would take longer to complete over time. Reading over a couple of pages, it seems that SSD degradation is pretty normal. After some time as the drive fills up it will become slow, and actually doing an UNMAP on SSD drives can help out:

Some notes, from the above sites:

Over time, our daily production traffic caused the SSDs to become fuller from their perspective. As a result, garbage collection was triggered more and more often until disk performance reached unacceptable levels. One natural way to solve the issue is to tell SSDs which data are deleted. Modern operating systems support an instruction called TRIM to allow file systems to pass the information to the underlying disks.

From another site:

As the number of known free Flash cells decreases the write performance of the SSD also decreases because it heavily depends on the number of cells that can be simultaneously written to.

To address this issue the ATA TRIM command was introduced many years ago. Modern Operating Systems use the TRIM command to inform the SSD controller when they delete a block so that it can add the associated Flash cell to its free list and knows that it can be overwritten.

And here is the last one:

It’s because of this relationship between write amplification and spare area that we’ve always recommended setting aside 10 - 20% of your SSD and not filling it up entirely. Most modern controllers will do just fine if you partition the drive and leave the last 10 - 20% untouched. With TRIM support even the partitioning step isn’t really necessary, but it does help from a data management standpoint.

SCSI UNMAP with VMware

On the local drive I confirmed that UNMAP is supported (Delete Status):

[root@macm:~] esxcli storage core device vaai status get -d t10.ATA_APPLE_SSD_SM256E
t10.ATA_APPLE_SSD_SM256E
   VAAI Plugin Name:
   ATS Status: unsupported
   Clone Status: unsupported
   Zero Status: supported
   Delete Status: supported

Then I ran the following to send an UNMAP to the VMFS Datastore:

[root@macm:~] esxcli storage vmfs unmap -l datastore1

That took about 2-3 minutes to complete, and in the logs I saw the following:

[root@macm:~] tail -f /var/log/vmkernel
2017-02-19T03:57:25.503Z cpu2:72098 opID=62289008)vmw_ahci[0000001f]: scsiUnmapCommand:Unmap transfer 0x18 byte
2017-02-19T03:57:26.019Z cpu2:72098 opID=62289008)vmw_ahci[0000001f]: scsiUnmapCommand:Unmap transfer 0x18 byte
2017-02-19T03:57:26.537Z cpu5:72098 opID=62289008)vmw_ahci[0000001f]: scsiUnmapCommand:Unmap transfer 0x48 byte
2017-02-19T03:57:27.052Z cpu5:72098 opID=62289008)vmw_ahci[0000001f]: scsiUnmapCommand:Unmap transfer 0x18 byte

Then my performance on the local drive came back. I was able to write a file pretty quickly:

[root@macm:/vmfs/volumes/533e29ae-e243ce90-39a5-685b35c99610] time vmkfstools -c
 15G -d eagerzeroedthick test.vmdk
Creating disk 'test.vmdk' and zeroing it out...
Create: 100% done.
real	0m 34.59s
user	0m 4.75s
sys	0m 0.00s

Before running the UNMAP command, the above took 3 minutes.

SCSI UNMAP with ZFS/Solaris

Reading over OmniOS r151014, it looks like with OmniOS version r151014 it can handle UNMAP, which is great:

New tunables:

  • zfs_free_max_blocks (can reduce for less free blocks per transaction)
  • zvol_unmap_enabled (can set to 0 to ignore UNMAP requests which can be slow)
  • metaslabs_per_vdev (an upper limit per vdev, currently 200, now tunable)

And I did confirm that the ESXi host can see the UNMAP Primitive (the Delete Status) for the volume/LUN presented over iSCSI with Comstar:

[root@macm:~] esxcli storage core device vaai status get -d naa.600144f070cc4400
naa.600144f070cc44000
   VAAI Plugin Name:
   ATS Status: unsupported
   Clone Status: unsupported
   Zero Status: supported
   Delete Status: supported

However when I sent the UNMAP command to the LUN it ran really fast and it felt like it didn’t do anything. I then ran into this interesting page: ZFS and Intelligent Storage. Which talks about how UNMAP with ZFS doesn’t work very well:

By sending UNMAP/TRIM commands, ZFS can notify the array that a particular block of storage is no longer required, which on most arrays will trigger the array to re-thin-provision that block of storage, freeing the space it was using.

Unfortunately that’s where the good news ends. Solaris/ZFS added support for UNMAP in the Solaris 11.1 release, however their implementation was so horribly broken that they recommended disabling it in the release notes for the very same version! In a release soon after they disabled it by default, and despite it now being almost 4 years and 2 Solaris release later, they still do not recommend ever turning it on.

Re-formatting the LUN and starting all over again gave exactly the same results - a few minutes into the random workload the pool once again reported data corruption - confirming Oracle’s advice that turning on UNMAP for ZFS is not a good idea!

I then remembered a recommendation from napp-it: Tuning/ best use

Do not fill up SSDs as performance degrades. Use reservations or do a “secure erase” on SSDs that are not new, followed by overprovision SSDs with Host protected Areas,

Formating SSD Drives in Solaris

I removed the LUN, I removed the volume, and I deleted the pool. Then I formatted the disks:

<> format
Searching for disks...done


AVAILABLE DISK SELECTIONS:
       0. c2t0d0 <ATA-ST3808110AS-H cyl 9726 alt 2 hd 255 sec 63>
          /pci@0,0/pci1043,8534@1f,2/disk@0,0
       1. c2t1d0 <ATA-ST1000DM003-1CH162-CC47-931.51GB>
          /pci@0,0/pci1043,8534@1f,2/disk@1,0
       2. c2t2d0 <ATA-ST1000DM003-1CH162-CC47-931.51GB>
          /pci@0,0/pci1043,8534@1f,2/disk@2,0
       3. c2t3d0 <ATA-ST1000DM003-1ER162-CC45-931.51GB>
          /pci@0,0/pci1043,8534@1f,2/disk@3,0
       4. c2t4d0 <Samsung-SSD 840 EVO 250GB-EXT0BB6Q-232.89GB>
          /pci@0,0/pci1043,8534@1f,2/disk@4,0
       5. c2t5d0 <Samsung-SSD 840 EVO 250GB-EXT0BB6Q-232.89GB>
          /pci@0,0/pci1043,8534@1f,2/disk@5,0
Specify disk (enter its number): 4
selecting c2t4d0
[disk formatted]

FORMAT MENU:                                                             [33/57]
        disk       - select a disk
        type       - select (define) a disk type
        partition  - select (define) a partition table
        current    - describe the current disk
        format     - format and analyze the disk
        fdisk      - run the fdisk program
        repair     - repair a defective sector
        label      - write label to the disk
        analyze    - surface analysis
        defect     - defect list management
        backup     - search for backup labels
        verify     - read and display labels
        inquiry    - show vendor, product and revision
        volname    - set 8-character volume name
        !<cmd>     - execute <cmd>, then return
        quit
format> analyze

ANALYZE MENU:
        read     - read only test   (doesn't harm SunOS)
        refresh  - read then write  (doesn't harm data)
        test     - pattern testing  (doesn't harm data)
        write    - write then read      (corrupts data)
        compare  - write, read, compare (corrupts data)
        purge    - write, read, write   (corrupts data)
        verify   - write entire disk, then verify (corrupts data)
        print    - display data buffer
        setup    - set analysis parameters
        config   - show analysis parameters
        !<cmd>   - execute <cmd> , then return
        quit

analyze> purge
The purge command runs for a minimum of 4 passes plus a last pass if the
first 4 passes were successful.
Ready to purge (will corrupt data). This takes a long time,
but is interruptible with CTRL-C. Continue? y

        pass 0 - pattern = 0xaaaaaaaa
   488397042  64  44

        pass 1 - pattern = 0x55555555
   488397042

        pass 2 - pattern = 0xaaaaaaaa
   488397042

        pass 3 - pattern = 0xaaaaaaaa
   488397042

The last 4 passes were successful, running alpha pattern pass
        pass 4 - pattern = 0x40404040
   488397042

Total of 0 defective blocks repaired.

Then I re-created the pool, volume, and LUN (more on the process check out the ZFS iSCSI Benchmark Tests on ESX post). Before re-adding it to the ESXi host I ran a quick bonnie++ and I got the following:

zfs-bon-results

Before running the purge with the format utility, I got 60MB/s Write and 700MB/s for Read. I am glad to see my write performance back.

Automatic UNMAP with vSphere 6.5

Then I read that with vSphere 6.5 and VMFS-6, UNMAP is automatic. From What’s new in vSphere 6.5 Core Storage:

This is a feature I know that many of you have been waiting for. There is now automatic UNMAP with VMFS-6 and vSphere 6.5. This automated UNMAP crawler mechanism will reclaim what is termed “dead” or “stranded” space on VMFS-6 datastores. Blocks that have been freed will be reclaimed within 12 hours by the crawler.

But unfortunately you can’t do an upgrade: Migrating VMFS 5 datastore to VMFS 6 datastore. So I moved everything off the local datastore and formatted it with VMFS-6. I also readded the ZFS LUN, formatted it with VMFS-6 from the get-go, and I was able to confirm that it’s enabled:

[root@core:~] esxcli storage vmfs reclaim config get -l datastore1
   Reclaim Granularity: 1048576 Bytes
   Reclaim Priority: low

BTW more information on that feature is here:

If you run that on a VMFS-5 datastore you will see the following:

[root@macm:~] esxcli storage vmfs reclaim config get -l  datastore1
Failed to retrieve unmap property for filesystem, VMkernel log may contain more details.
Reason: VMFS with version 5 does not support unmap property

Now hopefully with that enabled, the performance won’t degrade as bad (but still 4 years on an SSD is great)


Published by Karim Elatov

12 August 2017

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