Cache Enabled or Disabled?

[Myth]

Hi guys,

We have a 16 bay 6TB HDD RAID 6 array with seagate sas 12 64mb cache drives 7200rpm. In the adaptec controller software we have the option to set each drive as either write back or write through.

It is my understanding that write back is cache enabled setting. So I've set all sixteen drives to write back. I notice about a 200MBps improvement in write performance on the array using AJA speed test.

For some reason the default setting was write through. So I'm just wondering if there is any concerns in enabling write back? This array really needs read performance as it's an AVID server so it needs to play back footage without dropping frames. Would enabling the cache cause the read performance to diminish?

thanks for any advice

-Myth

 

[i386]

Disable it, there are no capacitors/batteries that protect the cache on hdds.

 

[Evan]

It all comes down to the risk of corrupting filesystems on writ during a power failure for example. Really depends how your workflow is and if the increased risk is worth the performance.

I don’t know adaptec controllers for years now but I would have assuming this setting is saying do we apply the cache on the controller, but I would have expected that at the raid group level not the disk level (I don’t think it’s refering at all to the 64k disk cache)

Does the controller have a bbu for the cache ? If yes then it’s safer, and I would switch it on.

 

[Evan]

Disable it, there are no capacitors/batteries that protect the cache on hdds.

You think it’s refering to the cache on disks or the controller ?

 

[i386]

I don’t know adaptec controllers for years now but I would have assuming this setting is saying do we apply the cache on the controller, but I would have expected that at the raid group level not the disk level (I don’t think it’s refering at all to the 64k disk cache)

There are setting for the controller cache and for the individual drives. If you create a logical volume it disables all the drive caches and depeding if you have a bbu/battery it will enable the controllers cache.

 

[Evan]

There are setting for the controller cache and for the individual drives. If you create a logical volume it disables all the drive caches and depeding if you have a bbu/battery it will enable the controllers cache.

Got it, have not see any adapec controllers for years so was a guess.
Didn’t know you can control at the disk level also but makes sense.

 

[Myth]

Yes that's correct, there is a read and write cache on the logical drive. Which i have enabled. If I disable that, then the write speeds on RAID 6 are less than 50MBps so I have to have that one on.

Then under each physical enclosure tab, there is a write back or write through setting.

The adaptec controller had a battery backup for it's onboard RAM cache, I think the cache size is 1024MB. But I don't know if the HDD cache goes through that Battery backup.

We do have a universal power supply which support graceful shutdown should the power fail across the entire building for a long peroid of time. But if the power fails for more than 5 minutes I imagine the workstations would start to shutdown before the server.

I know that sometimes UPS's don't work right, but this one is testing and working for a number of power blips.

 

[mrkrad]

You need to enable drive-write-back cache or your performance will suck! Using proper shutdown scripts/apps will protect you from most outages!

 

[i386]

You need to enable drive-write-back cache or your performance will suck!

This is dangerous: the drive will aknowledge writes buffered in the ram before they are persisted on the platters.

 

[mrkrad]

What if the drives support PLP with supercaps?

 

[i386]

Than it will work.

Had to google if such drives exist
The quick google search shows that only toshiba has some enterprise hdds with plp.

 

[mrkrad]

Most drives like samsung pm843,sm843, intel server-line come with PLP - in fact some brands of consumer drives come with the feature as well - not the cheapo brands.Maybe Micron?

 

[i386]

Op is asking about cache on hdds, not ssds.