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Slow write speed on intel Raid 5 (6xRED 4tb)


Hi all,

I hope someone can lend me a hand since now I have get out of ideas.

I've built a raid 5 on a Asrock z87 extreme6 using six Western Digital RED 4tb that are connected to the six intel controller SATA3 ports, with the aim of creating a 20Tb Raid 5. The OS is Windows 8.1 x64.

I created the raid from the BIOS utility selecting 64kb size (I had option of selecting 64 and 128 but the utility reccomended 64kb in case of raid 5). Once in Windows I formatted the raid unit with a 20Gb partition and write speed was really slow (10 MB/s max), even after waiting raid to be completely constructed (it took several hours)

After reading and looking for information I enabled write cache and disabled write-cache buffer flushing. I also set simultaneous on in the Intel Rapid Storage Technology panel. After doing this the write speed increased to 25-30 MB/s.

I have notice that physical cluster size is 4098 bytes (usual on those 4tb disks), but logical cluster size is 512 bytes:


Shouldn't those cluster sizes match to have a good performance? In this case, how to change it?

I've try to delete partition and create again, but selecting different cluster sizes for que partition, and the best performance is using 64kb (the stripes size), but it's only 50-60 MB/s actual speed copying a big MKV file from an SSD, and even doing it if doesn't makes any change on the capture where we see the 512 bytes for logical sector size.

AS SSD Benchmark seems to tell that partition is correctly aligned:


The results of the speed here seems ok, but as I told, real speed never exceeds 58-59 mb/s in writes.

I attack a capture of fdisk, I really don't know if it's ok or bad aligned:


ATTO DISK Benchmark:


Those 6 discs were installed on a NAS, having a write speed higher than 80 mb/s, where is the problem here?

Many thanks in advance

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5 Replies

Hi Tassadar,

Let me help you with this one.

I first want to thank you for sharing all those details with us but there is some more information I would like to ask you so that I can have a very and more clear image of your issue.

Please answer this:

  1. Was this working fine before? For how long have you been using this Raid 5?
  2. What is the NAS brand and model? Are you still running this RAID 5 with that NAS?

It is important to say that the cluster size depends on the file size you will handle. Here is a very good article you can check about it:

NOTE: These links are being offered for your convenience and should not be viewed as an endorsement by Intel of the content, products, or services offered there.,1735-4.html,1735-4.html

I can also confirm that rebuilding a RAID 5 with that amount of Disks and capacity of each drive will take some time so it may be expected if you find the rebuilding process slow.

Kevin M


I have the same problem on my backupserver. This is my configuracion:

Asrock H87 PRO4

Xeon 1220V3

12GB DDR3 1600 (no ECC)

64GB SSD (system) + 5X Caviar Red 4TB RAID5 (Data)


I have another with

asrock H87 PRO4

I3 4150;

4GB DDR3 1600 (No ECC)

120GB SSD (system) + 1X 4 TB RED (data)

this is the result:

Help please!


Hi jsgimenez,

Now from those images I see the system with the 60GB SSD is in raid and TRIM function works on RAID 0 only so this may be the reason why even having the SSD a system disk the write speed on the volume will be affected.

Now my question is, Do you have both systems on RAID?

Kevin M


hi kevin thanks for answering.

SSD (SYSTEM) is NO RAID, and is only system (windows). Raid 5 is only for data, no page file, no sistem, only data.

I have destroyed the RAID5 and I created back when finished initializing tell you if it works well.

Thank you.!

pd. srry my English.