Information Technology Reference
In-Depth Information
ability to survive a power loss) is critical for write caches because the data is not yet com-
mitted to disk, but it's not critical for read caches. Cached performance is often used when
describing shared storage array performance maximums (in IOPS, MBps, or latency) in speci-
i cation sheets. These results generally do not rel ect real-world scenarios. In most real-world
scenarios, performance tends to be dominated by the disk performance (the type and num-
ber of disks) and is helped by write caches in most cases, but only marginally by read caches
(with the exception of large relational database management systems, which depend heavily
on read-ahead cache algorithms). One vSphere use case that is helped by read caches is a situ-
ation where many boot images are stored only once (through the use of vSphere or storage
array technology), but this is also a small subset of the overall VM I/O pattern.
Disks
Arrays differ as to which type of disks (often called
spindles
) they support and how
many they can scale to support. Drives are described according to two different attributes.
First, drives are often separated by the drive interface they use: Fibre Channel, serial-attached
SCSI (SAS), and serial ATA (SATA). In addition, drives—with the exception of enterprise
l ash drives (EFDs)—are described by their rotational speed, noted in revolutions per minute
(RPM). Fibre Channel drives typically come in 15K RPM and 10K RPM variants, SATA drives
are usually found in 5400 RPM and 7200 RPM variants, and SAS drives are usually 15K RPM
or 10K RPM variants. Second, EFDs, which are now mainstream, are solid state and have no
moving parts; therefore rotational speed does not apply. The type and number of disks are
very important. Coupled with how they are coni gured, this determines how a storage object
(either a LUN for a block device or a i le system for a NAS device) performs. Shared storage
vendors generally use disks from the same disk vendors, so this is an area of commonality
across shared storage vendors. The following list is a quick reference on what to expect under
a random read/write workload from a given disk drive:
7,200 RPM SATA: 80 IOPS
◆
10K RPM SATA/SAS/Fibre Channel: 120 IOPS
◆
15K RPM SAS/Fibre Channel: 180 IOPS
◆
◆
A commercial solid-state drive (SSD) based on Multi-Level Cell (MLC) technology:
1,000-2,000 IOPS
◆
An Enterprise Flash Drive (EFD) based on Single-Level Cell (SLC) technology and
much deeper, very high-speed memory buffers: 6,000-30,000 IOPS
Bandwidth (Megabytes per Second)
Performance tends to be more consistent across drive
types when large-block, sequential workloads are used (such as single-purpose workloads
like archiving or backup to disk), so in these cases, large SATA drives deliver strong perfor-
mance at a low cost.
Explaining RAID
Redundant Array of Inexpensive (sometimes “Independent”) Disks (RAID) is a fundamental
and critical method of storing the same data several times. RAID is used to increase data avail-
ability (by protecting against the failure of a drive) and to scale performance beyond that of a
single drive. Every array implements various RAID schemes (even if it is largely invisible in i le
server persona arrays where RAID is done below the i le system, which is the primary manage-
ment element).
