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I'm looking to building a RAID, and ran into a thought.

what is better, fewer larger drives (1 TB x 3), or more smaller drives (500 GB x 6).

I was looking at specifically a RAID 5 or similar. Is there a difference to things like data integrity, random read/writes, etc.

EDIT: I'm sorry, I mis-spoke, I was looking at only RAID 5 (and forgot that it has a minimum of 3 drives).

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  • Homework? This is the sort of design vs. budget vs. other constraints that a sysadmin needs to be able to figure out. May 30, 2011 at 5:15
  • 1
    Hm, being told to do my homework, as I'm doing it. Odd.
    – Jess
    May 30, 2011 at 6:18
  • What are you going to use this for? Server? How much reliability is important? SAS or SATA?
    – Mascarpone
    Nov 12, 2011 at 20:31

6 Answers 6

8

When having the choice, go with more smaller drives instead a few large drives. This has different reasons:

  • You can stripe reads and writes over more drives, leading to a speed increase
  • Rebuilding smaller drives takes much less time.
  • Very fast (15K SAS) drives will not be available with larger capacities
  • Random reads could be handled by more heads, leading to a speed increase for applications requiring many small random reads.

Beware though that depending on your total capacity, a RAID5 could be considered unsafe, as encountering an URE while reconstructing a failed drive might become very likely.

Also, consider that using only two drives either gives you no redundancy or no speed increase, depending on your selected RAID type.

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  • Another factor to consider is that, all else being equal, smaller capacity drives tend to be more reliable. May 30, 2011 at 0:58
  • Not totally correct. Two drives do give you a speed increase during reads.
    – KCotreau
    May 30, 2011 at 1:28
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IMHO this question can not be answered correctly with the information provided. Prior to evaluating which RAID to implement one needs to have a specificition of the use. A RAID 5 implementation will satisify some requirements and fail to satisify others, as will other RAID implementations.

Layout all your requirements, performance, read write ratios, budgetg, etc and then decide or ask which implemention will suit your needs.

There are many WHITE papers on the web which describe the best use of each raid type.

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I would prefer RAID 5 there since you lose less space, and I consider the performance better. With 2x1TB RAID 1, you lose a full terabyte. With the RAID 5, you lose only 500GB.

That said, each has its own advantages.

RAID 1 advantages: All the data is redundant. If one disk fails, you do not have any performance degradation. Easy to understand.

RAID 1 disadvantages Lose more space. Does not perform as well as RAID 5. Has to write everything to both disks...no performance increase on writes.

RAID 5 performance is better for reads, which is good for database applications. Since you asked specifically about this, better at random read operations and writes since it can read/write from/to multiple disks, however this is also mitigated by having to update the parity information for each write, so the benefits are not as great as with reads. RAID 1 also can read from both disks, but limited to just two. These advantages can be increased by adding more disks. Easier to add more space if you run low.

RAID 5 disadvantages Performance degradation with the loss of a disk RAID has to be rebuilt when one fails and is then replaced.

There are surely more reasons that can be argued, but these are the main ones.

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  • Just one final point: Although no one solution is perfect for everyone, you have to look at how much more popular RAID 5 has been with IT professionals over RAID 1.
    – KCotreau
    May 29, 2011 at 23:24
  • Very well put, but I was looking at specifically the difference between large/few and small/many drives with a RAID 5. I misspoke, see the edited post
    – Jess
    May 30, 2011 at 0:46
  • OK. Then depending on the specific device and how many bays it has, and how much money the organization has, I would say the following...generally: If you have a large organization with unlimited money, a lot of small drives has more performance benefit, usually a major factor to such an organization. For a smaller organization, which may gain more data, but not have a lot of money to replace drives or servers, I would go with larger drives, leaving room in the extra bays to grow. I have had companies struggle with money when they ran out of space.
    – KCotreau
    May 30, 2011 at 1:30
  • @KCotreau: For both RAID1 and RAID5, read performance scales with the number of disks. RAID1 uses more disks per capacity so for a given capacity, RAID1 has better read performance. RAID1 can normally be extended to RAID1+0 to add capacity. Main drawback of RAID5 is the R-M-W penalty for writing small IOs, which means there are physically 2 read and 2 write IOs for every logical IO. This means higher latency (important for database redo logs) and lower total IOPS. A battery-backed write cache can mask this performance penalty.
    – Tom Shaw
    May 30, 2011 at 1:49
  • @Tom Shaw, "RAID1 has better read performance"?? Well since RAID 1 tops out at two disks, and RAID 5 can go well beyond three, if you want performance, a RAID 5 with many disks is the way to go. In general, your statement is incorrect. webhostingresourcekit.com/305.html
    – KCotreau
    May 30, 2011 at 12:07
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I support SvenW's answer, but there are some drawbacks to consider about using smaller drives.

  • The number of disk slots may be constrained so using smaller drives may limit total capacity and future upgrade options.
  • More drives use more power and create more heat, vibration and noise, potentially reducing overall reliability.

Also, with the advent of SSDs, there is a school of thought that you can combine the capacity benefits of large disks with the performance benefits of solid-state disk, reducing the need for small high-performance disks. See my answer about cache devices in ZFS.

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If it's for home use RAID 5 is fine, just remember, a RAID isn't a backup, and data isn't safe just for being in a RAID.

For a company just avoid RAID 5, RAID 5 is something almost any company will regret, mainly because the companies choosing RAID 5 over RAID 6 tend to also be companies trying to save money where they really shouldn't, meaning these drives will run into the ground for many many years.

Normally we buy the same drives to use for a RAID, coming from the same manufacturing line, into the same depot, with the same life expectancy and wear rates. This means that by the end of the life of the first drive all other drives are on the way out. This makes the following scenarios extremely likely:

  • A second drive fails during the rebuild, all data is lost.

  • Any of the other drives has bad-blocks anywhere, since it's a raid 5 there's no way to rebuild the missing blocks, this will insert data corruption, often silently, you will think your data is fine until you try to read it.

The bigger the drives, the more likely for you to encounter the above issues, especially if the raid is built using spinning rust (traditional HDDs).

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RAID 5 is old now. I would stay away from RAID 5. Go with either RAID 6 for which you'd need 2 parity drives. You also also go with RAID 10 if you have lot of drives. With Raid 10 you're combining the speed of RAID 0 but adding redundancy of RAID.

With regards to larger drives. Be careful as very large drives especially with Western Digital use what they called "SMR" which single type layering data to get more space on to the drive. However, "SMR" is terrible in RAID especially if you have to rebuild the RAID. That would also be an issue if you ever have to rebuild a very large drive i.e. 2TB drive. vs having to rebuild RAID on 500GB drive. Older model drives used "CMR" type of recording which leaves little bit of space between in between the data. Western Digital will advertise if a "RAID" drive is CMR or "SMR"

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