Storage Spaces and Parity - Slow write speeds
I've recently been playing around with Windows Storage Spaces on Microsoft Windows Server 2012 R2. They are fantastic. ReFS brings so many benefits over NTFS.
But it's half complete it seems.
I originally created a parity volume, as I assumed this would be quite similar to RAID 6. You have the option of having a write array, or write cache using SSD drives. I haven't done this at this stage. I'm currently using 6x6TB Western Digital 7200RPM drives.
After creating the very large volume, I started copying some data. I was copying the data over a 1gbit network interface, so I was expecting to see 100mb/s, or close to it.
At first, I did get 100mb/s. For a minute or so anyway. Then I saw the speed slowly drop to around 30-45mb/s. I thought this was rather strange.
I upgraded all the drivers on the server, mainly the network drivers, as I saw the network speed drop to around that level at the same time as well. However, this made no difference.
I then started to do some research to figure out what was going on.
What I saw was the following: The memory was increasing to a certain, pre-defined point, then it would stop. This indicated that the copying was actually being buffered to memory (write-cache). I assume this is happening because I used the default options when creating a parity drive without a SSD array. This creates a 2GB buffer in memory, which you can clearly see here.
Once the memory buffer, or write-cache is full, you can see the speed drop and the memory start writing the data to disk.
Annoying huh? One way to fix this is by using a cache array of SSD hard drives, but there is another fix.
In PowerShell, you can set the storage space to believe it has battery backup. This is like having battery backup on a raid card. First you need to get the friendly name of your storage volume.
The command is
Now set the power protected mode of the pool as follows
Set-StoragePool -FriendlyName Backup -IsPowerProtected $true
replace backup with the name of your storage pool.
Here it is set as $false
Here it is set as $true
Quite a difference.
**** I should warn you though that if your server crashes, or has a power failure, your storage space may become corrupt. Make sure you have a UPS in place ****
Like I said earlier, this can be improved with a SSD cache array.
Hopefully this helps someone out there.
*** UPDATED 15/12/2015 ***
I highly recommend you view the Fujitsu white paper on Storage Spaces here.
This tip made my day, converting a 15 hour copy to a 2 hour one. Many thanks.
Yep! Thanks man! Now local Backup of Photos is faster than upload to Flickr 😉