Write access time slow on RAID1

Question

I'm running MongoDB on my personal computer. I noticed that performance is much slower when the data are on my 2-recent-spinning-hard-drive-software-RAID1 than when they are on an older spinning hard drive without RAID.

Old drive, no RAID

Single operations:

> var d = new Date(); db.test.createIndex({ "test": 1 } ); print(new Date - d + 'ms');
251ms
> var d = new Date(); db.test.createIndex({ "test": "2dsphere" } ); print(new Date - d + 'ms');
83ms
> var d = new Date(); db.dropDatabase(); print(new Date - d + 'ms');
71ms

Whole test suite: 250s

 Recent drives, RAID1 

Single operations:

> var d = new Date(); db.test.createIndex({ "test": 1 } ); print(new Date - d + 'ms');
1220ms
> var d = new Date(); db.test.createIndex({ "test": "2dsphere" } ); print(new Date - d + 'ms');
597ms
> var d = new Date(); db.dropDatabase(); print(new Date - d + 'ms');
671ms
> var d = new Date(); db.dropDatabase(); print(new Date - d + 'ms');
1ms

Whole test suite: 700s

 Configuration files

In case it would be useful (I doubt it):

/etc/fstab

UUID=d719f337-d835-4688-baf2-3e29f147ff15 /               ext4    errors=remount-ro 0       1
# /home was on /dev/md0p3 during installation
UUID=def01643-c71e-47df-9dc8-67096243aee6 /home           ext4    defaults        0       2
# swap was on /dev/md0p1 during installation
UUID=d43319a8-92fb-437d-b576-ef964276cde none            swap    sw              0       0
/dev/sr0        /media/cdrom0   udf,iso9660 user,noauto     0       0

 UUID="dd8b1f05-c65b-42e1-a45e-0ef421faf1df" /mnt/bak ext4 defaults,errors=remount-ro 0 1

/etc/mdadm/mdadm.conf

# auto-create devices with Debian standard permissions
CREATE owner=root group=disk mode=0660 auto=yes

# automatically tag new arrays as belonging to the local system
HOMEHOST <system>

# instruct the monitoring daemon where to send mail alerts
MAILADDR root

# definitions of existing MD arrays
ARRAY /dev/md/0  metadata=1.2 UUID=3a0f91ae:51c48198:3d1e26ed:118a1938 name=bouzin:0

# This configuration was auto-generated on Sun, 24 Jan 2016 18:00:55 +0100 by mkconf

Question

From what I've read, write access on the RAID1 should be roughly equal to the write access on a single equivalent drive.

• Could this 5400/7200 factor explain the order of magnitude of difference in the tests above?

• Could it be better without the RAID?

• Any interesting test/benchmark I could run? For now, I only have Mongo shell tests, but they seem to point to the RAID, or the drives, rather than to Mongo itself. Is there some application-agnostic test I could run to identify anything?

• Could anything be wrong or suboptimal with the RAID configuration?

EDIT:

Important is, that I mixed 7200 rpm and 5400 rpm drives.

Answer 1

RAID1 will be slower than a single drive even the drive specs are equal.

The reason for this is while RAID1 improves reliability by performing every write to both drives, this same action reduces performance

RAID0 splits writes between 2 drives which improves performance by sharing the load but reduces reliability for the same reason.

RAID5 is a happy medium that results in better performance than a single drive as well as increased reliability as the failure of a drive won't make the array inoperable (although it will slow down substantially under these conditions).

The above is by no means an exhaustive list of RAID levels and their pros and cons nor a recommendation of any specific RAID level. For hardware recommendations https://hardwarerecs.stackexchange.com/ is a good choice.

Regardless of your method of benchmarking, to obtain accurate benchmarks you should run several tests and average the results and do so in single user mode when the system is not running other tasks as anything else will skew your results, likely resulting in higher iowait times than anticipated.

Another simple form of benchmarking would be to run dd with a sample file of a specific size. say you had (or created) a sourcefile of random data of X GB in size. you could then run time dd if=sourcefile of=target file

by using dd's bs= parameter you could run the test with different block sizes (see man dd) which might be useful for tuning depending on your needs/environment.

It should probably go without saying, but the quality of the drives you use for a RAID array is an important factor. Seagate's Barracuda Pro and WD's Gold datacenter drives are specified to less than 1 Unrecoverable Error in 10^15 bits read (about 125TB).

However, many other large capacity drives aren't at the higher spec. If you use a low-spec drive in a RAID, there's a good chance the rebuild will fail.

Answer 2

RAID1 will be, at maximum, as fast as the slowest drive in the array.

Even if you had 3 drives in RAID1, where two of them are enterprise SSDs and one is consumer HDD, you will get the speed of that HDD.

For those, who never used or saw RAID1 on 3 or more drives, here is a wiki excerpt (link):

RAID 1 consists of an exact copy (or mirror) of a set of data on two or more disks