How Fast is a disk ?

I've been providing some information on storage, so I thought I'd post it!

Published performance figures for disks, disk controllers , storage arrays, SAN’s, NAS and DAS are usually based around optimal conditions which rarely match real world.

For example the maximum throughput for a disk is usually quoted in bits whilst most

users relate to bytes. The maximum throughput for an individual disk is usually calculated for sequential io

( where reads and writes are contiguous ) for databases the majority of io is random. When calculating the throughput of a disk the spin speed is the most influential

factor, the faster the spindle speed, the greater the throughput.

Disk Maximum theoretical Sequential io

Maximum theoretical random io

15k SCSI/SAS/FC 434 17410K SCSI/SAS/FC 294 1257.2k SATA 181 74 In reality 80% of this figure is the achievable figure ( Ref: SQL Server 2000 Performance Tuning Technical Reference :: Microsoft Press ) Calculations from published figures for Seagate Cheetah and Barracuda disks ( average value of read and write ) Throughput for SQL Server ( Typical )

Disk Seq io

Throughput @64k random io

Throughput @8k

15k SCSI/SAS 347 21.7 Mbyte/sec 140 1 Mbyte/sec

10K SCSI/SAS 235 14.7 Mbyte/sec 100 0.78 Mbyte/sec

7.2k SATA 145 9 Myte/sec 59 0.46 Mbyte/sec

SQL Server random io are normally 8k, sequential generally 64k Turning this to raid arrays

Creating a 4 spindle raid 5 array would support the following io capacities/sec o ( available capacity = 3 x disk size )

Disk Type Random Write Random Read Sequential Sequential

Write Read Io Mbyte Io Mbyte Io Mbyte Io Mbyte

15k SCSI/SAS 140 1 560 4 347 21.7 1388 86.8 10K SCSI/SAS 100 0.78 400 3.12 235 14.7 940 58.8 7.2k SATA 59 0.46 236 1.84 145 9 580 36

Creating a 4 spindle raid 10 array would support the following io capacities/sec o ( available capacity = 2 x disk size, match for spindle count )

Disk Type Random Write Random Read Sequential

WriteSequential Read

Io Mbyte Io Mbyte Io Mbyte Io Mbyte 15k SCSI/SAS 280 2 560 4 694 43.4 1388 86.8 10K SCSI/SAS 200 1.56 400 3.12 470 29.4 940 58.8 7.2k SATA 118 0.92 236 1.84 290 18 580 36

Creating a 6 spindle raid 10 array would support the following io capacities/sec o ( available capacity = 3 x disk size, match for capacity )

Disk Type Random Write Random Read Sequential

WriteSequential Read

Io Mbyte Io Mbyte Io Mbyte Io Mbyte 15k SCSI/SAS 420 3 840 8 1041 65.1 2082 130.2 10K SCSI/SAS 300 2.34 600 6.24 705 44.1 1410 88.2 7.2k SATA 177 1.38 472 3.68 335 27 670 54

Creating a 8 spindle raid 10 array would support the following io capacities/sec o ( available capacity = 4 x disk size, match for read capacity )

Disk Type Random Write Random Read Sequential

WriteSequential Read

Io Mbyte Io Mbyte Io Mbyte Io Mbyte 15k SCSI/SAS 560 4 1320 8 1388 86.8 2776 173.6 10K SCSI/SAS 400 3.12 800 6.24 940 58.8 1880 117.6 7.2k SATA 236 1.84 472 3.68 580 36 1160 72

Notes: Most scsi raid controllers enable split reads on raid 1/10 ( I’ve assumed this is the same for SATA – but this may not be the case )

A 4 disk raid 5 array presents 4 spindles for read, thus a comparison must be shown for a raid 10 where 4 spindles are presented for read.

Notes:

The io rates are taken from the average of the average seek times and track to track times.

Disks actually support slightly more reads than writes, but for the purpose of this comparison assume read and write io are the same.

For the 15k disk the absolute figures would be 166 write / 181 read

One way to improve performance further is to short format disks, e.g. format a 300Gb disk to 150Gb. This technique should halve seek times, disks are sometimes formatted this way with the outer partitions being allocated for fast performing usage and the inside to slower operations. However, a disk head can only be one place at a time and this type of horizontal partitioning can cause inconsistent performance.

Cache on disks should be disabled in raid arrays for databases. Controller cache should ideally be all allocated to write, read cache is

counterproductive for a RDBMS which attempts to cache its main data and handle read ahead.

Write cache attempts to turn random writes to sequential writes and/or facilitate elevator sorts – these increase write performance.

Note that the “large” amounts of cache memory on a SAN rarely improves

performance and a cache once saturated cannot improve disk performance. o 64Gb of cache divided read/write across 32 LUNs is actually not much memory at all

( A quick note about raid 6 .. This is raid 5 with an additional parity write, this means the array can stand a double disk failure, read performance remains unaffected but write performance degrades further due to the additional parity write. Raid 6 has a slightly better failure rate than raid 5 where there are large numbers of spindles in the array. )