Today only, Dell Home offers the Western Digital Caviar GP 2TB Serial ATA 3Gb/s Internal Hard Drive, model no. WD20EARS, for $99.99 with free shipping.

 

http://dell.to/aZEEs2

 

Thanks for posting... Was needing 2tb for my first unraid build.

 

Is there any reason to believe this wouldn't make a good parity drive?

Pick a 7200 rpm for a parity drive.  The write speed of the parity drive should be faster than all the data drives.

That's only true if you plan on doing simultaneous rights. If you're only ever writing to one drive at a time, then having a faster parity than data drive gives no benefit at all. The parity drive should be "as fast as" your fastest data drive if you want performance.

That's only true if you plan on doing simultaneous rights. If you're only ever writing to one drive at a time, then having a faster parity than data drive gives no benefit at all. The parity drive should be "as fast as" your fastest data drive if you want performance.

 

That seems so counter intuitive.  Content is written to the data drive then read from the data drive and the parity drive...calculated... and written back to the parity drive.

It would seem to me that the longer the write to the parity drive takes the slower the process will be. 

 

It's intuitive if you understand what happens behind the scenes.

 

You're bound by the slowest drive. What is done on the parity disk is also done on the data disk in parallel.

 

I'll rehash this discussion once again, skipping the part where the drives heads have to wait for a full rotation to reposition the drive heads for writing. Steps 1A/1B and 4A/4B, the reading/writing of the data and parity are done in parallel.

 

1A. Read current Data Content from Data Drive

1B. Read current Parity Content from Parity Drive

 

2. Wait for steps 1A and 1B to finish.

 

3. XOR out current Data Content from current Parity Content; XOR in new Data Content to calculate new Parity Content

 

4A. Write current new Data Content to Data Drive

4B. Write current new Parity Content to Parity Drive

 

5. Wait for steps 4A and 4B to finish.

 

 

If you don't trust my words, here's the words of "Joe L." [ http://lime-technology.com/forum/index.php?topic=5831.msg55078#msg55078 ]

 

The operations involved in unRAID are:

 

A1) Read existing sector X from Data Drive

A2) Read existing sector X from Parity Drive

 

B) Calculate new parity checksum value to write

 

Rotate data drive disk platter a full revolution to position read/write head over sector X again for writing

Rotate parity drive disk platter a full revolution to position read/write head over sector X again for writing

 

C1) Write new sector to Data Drive

C2) Write new sector to Parity Drive

 

Rotate data drive disk platter a full revolution to position read/write head over sector X+1 for reading

Rotate parity drive disk platter a full revolution to position read/write head over sector X+1 for reading

 

Since the disks must rotate a full revolution between each read and subsequent write of a sector when writing to the array, the "bottleneck" is the time it takes to physically rotate the disk. 7200 RPM drives rotate 120 times per second.  5400 RPM drives rotate 90 times per second, they will therefore introduce a rotational delay of between 1/60th and 1/45th of a second per stripe involved. (two rotations to read/write a sector and position to read the next).  The slowest rotation speed disk involved dictates the overall speed you can write to the array. 

 

Finally, unRAID operates on contiguous sets of sectors, referred to as a "stripe"  A full "stripe" is read/written at a time. 

The stripe size is configured to be less than the size of an entire cylinder on a disk.  This is for efficiency, A stripe will contain many sectors.

Therefore, a "stripe" is read from both data and parity drives, parity calculated, the platters rotate to position the disk head to write the entire stripe, then the platters rotate to read the next stripe, etc...

thanks for the info BRiT

 

FWIW

 

Newegg has this on sale $99 after $20 rebate:

HITACHI Deskstar HD32000 IDK/7K (0S00164) 2TB 7200 RPM 32MB Cache SATA 3.0Gb/s 3.5" Internal Hard Drive

http://www.newegg.com/Product/Product.aspx?Item=N82E16822145276

 

My question is now, how much does the cache play into the value of the drive inside an unraid system?

Not much.  More cache means faster writes, but the difference between 32 mb and 64 mb isn't going to be very noticeable.

Not much.  More cache means faster writes, but the difference between 32 mb and 64 mb isn't going to be very noticeable.

Especially when we typically write files that are multiple Gig in length.

either way, i ordered 3. got a call from dell this morning that the items have been backordered. i still think it was a good deal $0.055GB (although i did see 1.5TB for $0.053 today)