I'm not aware of any changes in default disk cache policy. If you are using unsupported drives I'm not sure what they would default.
The information you found regarding disk cache use being discouraged is correct. It is recommended that disk cache be disabled because it is not battery backed and puts your data at higher risk.
If you are concerned with performance then I suggest using a performance RAID controller. The performance increase achieved with enabling disk cache is negligible when a performance RAID controller is being used.
The disks are Dell Certified and supplied in an MD1400 on a PERC H830 (which is the highest performance RAID controller Dell offers). They are HGST HUH721010AL4200 10TB 4Kn.
If I use OpenManage to create a RAID6 virtual disk using the wizard in advanced mode the default for "Disk Cache Policy" is Enabled.
After reading the spec sheet on the HGST drive I don't believe there is anything volatile in it anymore. Likewise for the Seagate equivalent. It may be when the drives power up again if there's anything unwritten in cache it gets written.
If I use OpenManage to create a RAID6 virtual disk using the wizard in advanced mode the default for "Disk Cache Policy" is Enabled.
That sounds normal. As you noted previously, the default is to disable disk cache for SAS drives. The drive you are using is not a SAS drive, it is a Near Line SAS drive.
Enabling or disabling disk cache on a SATA or near line SAS spindle drive can have very noticeable performance impacts due to the slow access times. The performance difference is not as noticeable on SAS drives which typically have much faster access times. SATA and near line SAS drives for our servers are usually 7200 RPM. Supported SAS drives are generally 10k RPM or faster.
You should notice a performance increase by enabling disk cache. You can make the determination whether or not the performance increase/decrease of enabling/disabling disk cache is worth increased/decreased risk to your data.
The much longer specification (373 pages) mentions lots about volatile and non-volatile caches but never references something called "NVC quick cache". However on page 37 you find this:
6.4.1 Data integrity When the write cache option is disabled, no customer data is lost during power loss. If the write cache option is active, some data loss can occur during power loss.
If you google "seagate advanced write caching" you will find a pdf describing how their NVC system uses back EMF from the spindle motor when it's spinning down during a power failure to transfer the volatile cache contents to non-volatile cache. But how do you verify your particular Seagate drive has this capability?
how do you verify your particular Seagate drive has this capability?
If you post our part number from the drive I will check what information we have on it. There should be a 5 digit part number and a 25 digit PPID. I can use either of those to look up the drive.
I don't see any mention of NVC or non-volatile cache in the specs of our version of the drive. The specifications of our version differs from the listed specifications on Hitachi's site. For instance, our drive has 128MB cache. The specifications on their site list 256MB. The Hitachi part number of our version of the drive is 0F27439.
It does not appear that the drive has non-volatile cache.
Here's a pic of the label. It shows both the HGST and Dell part numbers. Of course the Dell version has Dell's version of the firmware, not HGST OEM firmware.
Daniel My
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October 24th, 2017 14:00
Hello
I'm not aware of any changes in default disk cache policy. If you are using unsupported drives I'm not sure what they would default.
The information you found regarding disk cache use being discouraged is correct. It is recommended that disk cache be disabled because it is not battery backed and puts your data at higher risk.
If you are concerned with performance then I suggest using a performance RAID controller. The performance increase achieved with enabling disk cache is negligible when a performance RAID controller is being used.
Thanks
avhjr
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October 24th, 2017 14:00
The disks are Dell Certified and supplied in an MD1400 on a PERC H830 (which is the highest performance RAID controller Dell offers). They are HGST HUH721010AL4200 10TB 4Kn.
If I use OpenManage to create a RAID6 virtual disk using the wizard in advanced mode the default for "Disk Cache Policy" is Enabled.
Daniel My
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October 24th, 2017 16:00
If the drive you are using does not have volatile cache then there is no reason to disable cache. I would leave it enabled.
avhjr
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October 24th, 2017 16:00
After reading the spec sheet on the HGST drive I don't believe there is anything volatile in it anymore. Likewise for the Seagate equivalent. It may be when the drives power up again if there's anything unwritten in cache it gets written.
Daniel My
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6.2K Posts
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October 24th, 2017 16:00
That sounds normal. As you noted previously, the default is to disable disk cache for SAS drives. The drive you are using is not a SAS drive, it is a Near Line SAS drive.
Enabling or disabling disk cache on a SATA or near line SAS spindle drive can have very noticeable performance impacts due to the slow access times. The performance difference is not as noticeable on SAS drives which typically have much faster access times. SATA and near line SAS drives for our servers are usually 7200 RPM. Supported SAS drives are generally 10k RPM or faster.
You should notice a performance increase by enabling disk cache. You can make the determination whether or not the performance increase/decrease of enabling/disabling disk cache is worth increased/decreased risk to your data.
Thanks
avhjr
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October 24th, 2017 17:00
Conflicting info. The drive data sheet describes "NVC quick cache" which is non-volatile.
www.hgst.com/sites/default/files/resources/Ultrastar-He10-DS.pdf
The much longer specification (373 pages) mentions lots about volatile and non-volatile caches but never references something called "NVC quick cache". However on page 37 you find this:
6.4.1 Data integrity
When the write cache option is disabled, no customer data is lost during power loss. If the write cache option is active, some data loss can occur during power loss.
www.hgst.com/sites/default/files/resources/HGST-Ultrastar-He10-SAS-OEM-Spec_r1.9.pdf
So I don't know anymore what to think esp. since it's enabled by default in OpenManage.
avhjr
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October 25th, 2017 06:00
If you google "seagate advanced write caching" you will find a pdf describing how their NVC system uses back EMF from the spindle motor when it's spinning down during a power failure to transfer the volatile cache contents to non-volatile cache. But how do you verify your particular Seagate drive has this capability?
Daniel My
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October 25th, 2017 09:00
If you post our part number from the drive I will check what information we have on it. There should be a 5 digit part number and a 25 digit PPID. I can use either of those to look up the drive.
Thanks
avhjr
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October 25th, 2017 15:00
Part number from OpenMange of one of the HGST 10TB drives:
TH0YG2KH1256768N02GTA00
Product ID:
HUH721010AL4200
Serial:
7PGKTTRG
Daniel My
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October 25th, 2017 18:00
I don't see any mention of NVC or non-volatile cache in the specs of our version of the drive. The specifications of our version differs from the listed specifications on Hitachi's site. For instance, our drive has 128MB cache. The specifications on their site list 256MB. The Hitachi part number of our version of the drive is 0F27439.
It does not appear that the drive has non-volatile cache.
Thanks
avhjr
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October 25th, 2017 18:00
Here's a pic of the label. It shows both the HGST and Dell part numbers. Of course the Dell version has Dell's version of the firmware, not HGST OEM firmware.
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