The cache disk is there to allow faster writing to an unraid system. Wouldn't it be possible to add an option so one could use a number of disks to build a RAID-0 cache disk? ( SSD's are expensive.. )

The cache disk is there to allow faster writing to an unraid system. Wouldn't it be possible to add an option so one could use a number of disks to build a RAID-0 cache disk? ( SSD's are expensive.. )

You can do this now with some MB that allow hardware raid-0 for the cache drive.  unRAID will not be able to spin it down, or get the disk temperatures if I remember correctly.

 

You will be limited by the network regardless as far as the "write" speed...  The raid-0 cache is NOT for speed, but to protect your data in case the cache drive crashes before it is written to the main protected array.

 

Yes, the "cache" disk was originally added to unRAID to improve the "write" speed, but that was when the "write-speed" was to the protected array was around 8 to 10 MB/s.  That was not fast enough for some unRAID users recording live TV streams.

 

Today, with faster hardware and vastly improved linux kernels, you can get sustained 30MB/s write speed to the protected array.  The need for a cache drive is just about eliminated for all but a very few who need to be able to write as fast as their network will allow.  (40-50MB/s possibly) 

 

Yes, the "cache" disk was originally added to unRAID to improve the "write" speed, but that was when the "write-speed" was to the protected array was around 8 to 10 MB/s.  That was not fast enough for some unRAID users recording live TV streams.

 

It's common to confuse MB/s (MegaBytes per second) with mbps (megabits per second).

 

There are no TV streams anywhere that are 8MB/s (64mbit/sec). Even wild MPEG2 satellite feeds top out at 30 to 40mbit/sec (I have a few of them).

 

You'd have to be recording around 5-10 simultaneous off-air HDTV streams to break 8MB/sec.

Yes, the "cache" disk was originally added to unRAID to improve the "write" speed, but that was when the "write-speed" was to the protected array was around 8 to 10 MB/s.  That was not fast enough for some unRAID users recording live TV streams.

 

It's common to confuse MB/s (MegaBytes per second) with mbps (megabits per second).

 

There are no TV streams anywhere that are 8MB/s (64mbit/sec). Even wild MPEG2 satellite feeds top out at 30 to 40mbit/sec (I have a few of them).

 

You'd have to be recording around 5-10 simultaneous off-air HDTV streams to break 8MB/sec.

Oops... sorry for any confusion, but then I did not mention the network data rate.  I was referring to disk write speeds.

 

The basics of what I was trying to say are still valid.  Originally unRAID was unable to keep up with high bit-rate feeds being written to the protected array.  It was when we only had IDE drives and a single PCI bus, and a much older Linux kernel.  The cache drive was added to address the "write" speed problem.   

 

Today, it is possible to write to the protected array at three times the rate originally possible.  The cache drive is only needed for those trying to wring every bit of speed out of their server when writing to it.  (It has no effect when reading)

The raid-0 cache is NOT for speed, but to protect your data in case the cache drive crashes before it is written to the main protected array.

 

Don't confuse RAID-0 (striped concatenation) with RAID-1 (mirroring).  A RAID-0 cache drive can increase read performance, but adds the danger of a single drive failure destroying all data on the array.  RAID-1 on the cache would protect the cache drive from failures.

In summary - it is possible, but there's not much of a point to it.

 

SSDs also don't provide much of a benefit as a cache drive.  In my tests, both my SSDs were at most 5 mb/s faster than my regular old 7200 rpm drive (65 mb/s vs 70 mb/s, if I remember correctly).

In summary - it is possible, but there's not much of a point to it.

 

SSDs also don't provide much of a benefit as a cache drive.  In my tests, both my SSDs were at most 5 mb/s faster than my regular old 7200 rpm drive (65 mb/s vs 70 mb/s, if I remember correctly).

 

This is why I believe the cache drive as large as a parity drive for warm spare is the best course of action.

The only reason I do not do this is my RAID0 Parity and RAID1 Cache are on the same pair of drives.

 

80% RAID0 for parity, 20% RAID1 for cache and internal protected application space. (and swap).