In RAID 10 with four 500GB HDD's, how much storage space will I have?
If I have a RAID 10 setup with four 500GB hard drives, how much effective storage will I have? And please don't redirect me to the FAQ or something... just give me the answer because I have read everywhere and I get answers differing from 500GB to 1TB.
Its not so much RAID 10 as it is RAID 1+0, if you see it that way, u get whats going on. You start with 4 drives, and since its 1+0, its raid 0 first, thus you get 2 RAID 0 units each consisting of 2 500GB drives with the equivalent size of 1TB. Then its a 1, so the 2 1TB units are then put into a RAID 1. So your final HD space will be around 1TB.
this setup to me is actually not optimal, as your fault tolerance is only 1 drive. On the second failure, you loose your array. (if 1 drive fails, half your RAID 1 fails, the working disk in that branch is disreguarded until a spare is put in. Thus the failure of any drive on the other half results in data loss)
However if your setup is RAID 0+1, you are allowed up to 2 drives failing without loosing data. (if 1 drive from each RAID 1 fails, the RAID 0 still stands).
So i would recommend RAID 0+1 or or RAID 01, same number of drives, same total size, one's safer.
look at this picture: http://www.sohoconsult.ch/raid/images/raid10.gif
This picture apparently explains how RAID 10 works with 16 HDD's. Can somebody please explain this to me a bit further? I still don't understand how I will have 1TB storage after looking at that diagram.
You're making it far more complicated than it needs to be. RAID 10 is a combination of RAID 1 and RAID 0. With RAID 1 you mirror 2 drives and wind up with the same storage as a single drive but with fault tolerance. With RAID 0 you stripe 2 drives for speed, and get the total sum of both drives, but losing either drive will destroy all the data.
I'm fuzzy on 0+1 vs 1+0 but the idea is basically the same, just in a different order, and admittedly with somewhat different levels of fault tolerance. However, depending on what motherboard you are using you probably don't have a choice in the matter since most motherboards support one or the other. Since you're new to RAID I'm going to assume you're not going to shell out hundreds of dollars for a dedicated RAID card... but that'll by far get you the best I/O and throughput performance.
Anyhow, what you'll be doing is creating 2 mirrored sets and striping them or vice versa.
So: [(500GB+500GB mirrored=500GB) + (500+500GB mirrored=500GB) striped]= 1000GB or
[(500GB+500GB striped=1000GB) + (500GB+500GB striped=1000GB) mirrored] =1000GB
As you can see, either way you wind up with 1000GB (which formatted will yield about 931GB due to the decimal to binary conversion)
Draw a vertical line down the middle of this illustration,
right through the "m" in "Performance":
On the left are mirrored data: a "chunk" of data named "A"
gets written to both spindles; then, the next "chunk" named "B"
gets written to both spindles; and so on, until the entire
file is written to both, resulting in two (2) copies of the file:
one copy on one spindle and a second copy on the other spindle.
On the right are striped data: a "chunk" of data named "A"
gets written to one spindle; then, the next "chunk" of data named "B"
gets written to the other spindle; then, the next "chunk" named "C"
gets written to the same spindle as the "A" chunk; then,
the next "chunk" named "D" gets written to the same spindle
as the "B" chunk; and so on.
Mirroring (RAID 1) can only go as fast as a single spindle;
striping (RAID 0) goes faster than a single spindle, but
never reaches twice as fast because of rotational
latencies, armature seeks, and other sources of
overhead such as controller efficiency (or lack thereof).
Even with solid-state disks, which eliminate rotational latency
and recording head movements, "scaling" is not linear when
the number of drives in a RAID 0 array is increased from
2 to 4 to 8 devices. Thus, a RAID 0 array with 2 devices
will generally NOT operate twice as fast as a single device,
but at a rate significantly less than twice as fast.
Lastly, a RAID 10 set is merely 2 x RAID 0 arrays that are also mirrored
i.e. both RAID 0 arrays have identical copies of exactly the same data.
raid 1+0 and raid 0+1 have the same chances of surviving. you can loose 1 drive, and possibly one more in each setup.
Performancewise you should be able to get almost 4 times the performance of a single disk, for sequential reads, if you use the software raid10,f2 from Linux. Read more on Linux raid performance on http://linux-raid.osdl.org/index.php/Performance and Linux raid in general on http://linux-raid.osdl.org