arik010
1 Nickel

disk cache policy in perc h700

Hello

could anyone spill some lighn on the DISK CACHE POLICY option on perch h700 configuration

i have 6XNEAR LINE SAS aray with write back and adaptive read ahead mode

does enabling the disk cache will enhance performace of the lun?

are the any disadvantages to do such thing? i didn't find anything about it in the manual.

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9 Replies
Moderator
Moderator

Re: disk cache policy in perc h700

Hello arik010

The virtual disk cache policy has three options, and here are the explanations for each:

Always read ahead: The controller will sequentially read ahead and write the data into cache for faster access. If the data being accessed is always random then there will be no performance benefit to this setting.

No read ahead: The controller will access the data on the array as it is requested.

Adaptive read ahead: If the previous two data requests were sequential then the controller will start proactively writing the data to cache for faster access.

The default is adaptive read ahead. There should be a slight performance increase when read ahead is used. It allows one step to be skipped in the process of accessing data by having the data already available in the controller's cache.

Thanks

Daniel Mysinger
Dell EMC, Enterprise Engineer

Get support on Twitter @DellCaresPRO

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arik010
1 Nickel

Re: disk cache policy in perc h700

thanks but this is not what i asked

imentioned that i already had adaptive read aheah mode enabled...

beside of that there is also something called disk cache policy that can be ticked enabled, probably reffering to the cache setting on the physical disks...

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arik010
1 Nickel

Re: disk cache policy in perc h700

anybody ?

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Moderator
Moderator

Re: disk cache policy in perc h700

beside of that there is also something called disk cache policy that can be ticked enabled, probably reffering to the cache setting on the physical disks...

Yes, that is what it is referring to. The setting within the operating system does not control the use of the HDD cache. The cache policy on the individual drives are controlled by that setting within the controller. I would recommend not enabling this disk cache. In the event of a power loss any data in the HDD cache will be lost and could cause data corruption. Enabling this option makes the battery backed feature of the cache on controller pointless.

Daniel Mysinger
Dell EMC, Enterprise Engineer

Get support on Twitter @DellCaresPRO

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arik010
1 Nickel

Re: disk cache policy in perc h700

thanks for clearing that up.

i set it to enbaled and there was actually huge improvment in past couple of days.

now i will have revert i to disabled in order to keep the system safe from data corruption 😞

does anyone here actually expirenced data corruption because of the local disk cache?

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pcmeiners
4 Tellurium

Re: disk cache policy in perc h700

If you have a decent external battery backup unit, meaning over sized battery backup unit, and you maintain the batteries, there is little chance of data loss. To go a step further, on my large server room setups, I have two BBUs, each one feeding one power supply of each dual power supply server, making near impossible to have a complete power loss affecting the physical disks due to a battery failure. 

As far as data loss in general, many years ago, as in early 1990s, rarely were raids run with battery backed cache or BBUs ( both were very expensive), caching was enabled, I ran several system as such, as did associates, we never had data loss, and these raids systems frequently lost power ..... now I would not think of running  without a BBU and BB cache, as both options are affordable and server are more utilized.     

carloscolombo
1 Copper

Re: disk cache policy in perc h700

Isn't it possible to enable HDD cache just for read-ahead and have the (H700 perc, battery backed) controller doing write-back caching at the same time? That would seem an ideal approach to me and it should be theoretically possible.

 

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Moderator
Moderator

Re: disk cache policy in perc h700

Isn't it possible to enable HDD cache just for read-ahead

No, the HDD cache policy is simply on or off. You cannot specify how it is used. You can specify the controller read and write policies. For example, you can have have adaptive read ahead enabled but have write through caching mode for write cache.

Thanks

Daniel Mysinger
Dell EMC, Enterprise Engineer

Get support on Twitter @DellCaresPRO

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agrisv
1 Copper

RE: disk cache policy in perc h700

in controller have 3 options:
read-policy( no read ahead, read ahead, adaptive read ahead)

write - policy (Write Through, Write Back,Force Write Back)

Disk Cache Policy (

  • Default — The disk used its default write cache mode.
  • Enabled — The controller reads the cache information to check if the requested data is available in the cache before retrieving the data from the disk. Reading the cache information first can provide faster read performance because the data (if available in the cache) can be quickly retrieved from the cache than from the disk.
  • Disabled — The controller retrieves data directly from the disk and not from the cache.)

Question about Disk Cache policy:

By default then create new virtual disk it selects Default(The disk used its default write cache mode.)

I know that is old "disk cache" on other conrollers where use it.
But in this Disk Cache policy, if i set to Enabled, then as i understand it is only user read data from disk cache?

It is safe from power issues? it is used for writes too?

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