COMP25212 ARRAY OF DISKS Sergio Davies [email protected] Feb/Mar 2014COMP25212 –...
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Transcript of COMP25212 ARRAY OF DISKS Sergio Davies [email protected] Feb/Mar 2014COMP25212 –...
COMP25212COMP25212
ARRAY OF DISKSARRAY OF DISKS
Sergio DaviesSergio [email protected]@manchester.ac.uk
Feb/Mar 2014 COMP25212 – Storage 2
Problems With DisksProblems With Disks
They are (were?) too small– Fixed (multiple disks), fixed (striping)
They sometimes fail– Fixed (mirroring), worsened
(multiple/striping)
They are too slow– Fixed (mirroring/striping)
2Feb/Mar 2014 COMP25212 – Storage 2
Storage 2: RAIDStorage 2: RAID
• Learning Objectives– To understand the technology drivers leading
to RAID arrays– To understand the principles of common
RAID configurations– To understand performance and reliability
consequences of common RAID configurations
– To understand the limitations of RAID failure mode operation and recovery
3Feb/Mar 2014 COMP25212 – Storage 2
Technology TrendsTechnology Trends
• 1980: Hard disk state of art: 1Gbytes in 14” removable drive
• 1980’s – most PCs used floppy disks
• 1990’s – most PCs used hard disks
• over 1 decade, hard disk sales volumes increased x100
• First effect: price reduction
• Second effect: PCs became drivers of disk technology
• Disk capacity and costs exceed Moore’s Law
4Feb/Mar 2014 COMP25212 – Storage 2
1985-1995 (approx)1985-1995 (approx)
• Two distinct markets for disk drives:– Mainframe/minicomputer/servers
(14 inch, high capacity, expensive)– PC
(8 inch to 5.25 inch to 3.5 inch; lower capacity, cheap(er))
• “How do we build server-class storage using PC-class components?”
5Feb/Mar 2014 COMP25212 – Storage 2
RAID in c.1990RAID in c.1990
• PC disks are small => use lots of them• PC disks are unreliable – how can we use
lots to increase reliability• Clue is in the “R” – “Redundant”• Use “Redundancy” to provide some error
immunity• We’ve already seen this in “Disk Mirroring”
– see Storage 1• What else can we do?
6Feb/Mar 2014 COMP25212 – Storage 2
A Diversion into A Diversion into MarketingMarketing
1985:
• Redundant
• Array of
• Inexpensive
• Disks
7Feb/Mar 2014 COMP25212 – Storage 2
A Diversion into A Diversion into MarketingMarketing
2010:
• Redundant
• Array of
• Inexpensive - Independent
• Disks
• Don’t expect “inexpensive”!!!
8Feb/Mar 2014 COMP25212 – Storage 2
RAID 0RAID 0
• Striping – (as per previous lecture)
9All images © Colin M.L. Burnett
Feb/Mar 2014 COMP25212 – Storage 2
RAID 3RAID 3
• Striping of (eg) bytes, with parity
• Can operate correctly with signalled errors
12Feb/Mar 2014 COMP25212 – Storage 2
RAID 4RAID 4
• Striping of blocks, with parity
• Performance issues:
1-block writerequires R-M-W
Excessive trafficon Disk 3
13Feb/Mar 2014 COMP25212 – Storage 2
RAID 5RAID 5
• Parity is distributed across multiple drives
• Distributes R-M-W
14Feb/Mar 2014 COMP25212 – Storage 2
RAID combinationsRAID combinations
• Each RAID configuration has its own advantages and disadvantages;
• Can we combine the advantages by combining RAID configurations?
• Multiple RAID layers, or nested RAID
16Feb/Mar 2014 COMP25212 – Storage 2
RAID 01 (0+1)RAID 01 (0+1)
• Balancing performance and redundancy
• Mirroring couple of striping disks
[ image from:http://www.linux-mag.com/id/7928/ ]
17Feb/Mar 2014 COMP25212 – Storage 2
RAID 10 (1+0)RAID 10 (1+0)
• Striping between mirrored couples of disks
• Same as before(?)
[ image from Wheart available at: http://commons.wikimedia.org/wiki/File:RAID_10.svg ]
18Feb/Mar 2014 COMP25212 – Storage 2
RAID 10 vs 01RAID 10 vs 01
[ image from Paulish available at: http://en.wikipedia.org/wiki/File:Raid_0%2B1.svg ]
[ image from Paulish available at: http://commons.wikimedia.org/wiki/File:RAID_10_6Drives.svg ]
19Feb/Mar 2014 COMP25212 – Storage 2
Let’s check the combinations of faults which may happen
RAID 50RAID 50
• High performance and error correction
20Feb/Mar 2014 COMP25212 – Storage 2
[ image from: http://blog.iweb.com/en/2010/05/an-overview-of-raid-technology/4283.html ]
RAID 160RAID 160
[ image from: http://www.linux-mag.com/id/7932/ ]
21Feb/Mar 2014 COMP25212 – Storage 2
On Failure – An Entire On Failure – An Entire DiskDisk
• (RAID 2-6)• Operate in degraded mode (every OS read
needs to read every disk)
• Replace drive (hot-swap?)
• Rebuild array – how long?– Sequentially– On live system?
• Failure during rebuild?
22Feb/Mar 2014 COMP25212 – Storage 2
Where to Implement Where to Implement RAID?RAID?
a) in OS
b) in Device Interface (RAID controller)– OS-independent– but how to provide User Interface?
23Feb/Mar 2014 COMP25212 – Storage 2