Regarding RAID 5 in LVM

[Shailendran V Naidu]

RAID-5 in LVM ......

How many times have I thought about that one !?
Couple of Hundred.

Answer is No.


Even for a plain striping we use PVGs and all the crazy stuff .. !!

[Lijeesh N G_1]

Hi,

Hardware RAID
================
>>By the name itself indicates that we are performing at Hardware level.
>>There is a controller(some time it will be on board or additional PCI card)called RAID controller which will provide this RAID solution.
>>We will create 1st RAID through RAID controller and will present to the OS as a single disk.RAID might be contains more than one disk,but OS will not be knowing that.If you take RAID5 at hardware level, it contains atleast 3 disks(or more),but from OS view it would be a single disk or LUN.

>>Failure of the RAID disk does not effect or no need to recover from OS,ie,Its not managing from OS at all.

Software RAID
===============
>>By the name itself indicates that we are performing at Software leve,ie OS level through some Disk management utilities(LVM,VxVM,etc.).
>>In this case OS will be managing the RAID,failure of the Managing utilities will effect the OS.


Generally we can say that,
Hardware RAID is hidden from OS and Software RAID is hidden from Hardware.

You may check following links for comparison,advantages,etc between them,

http://www.meganet1.com/pdf/Comparing%20Hardware%20vs%20software%20RAID.pdf

http://www.redhat.com/docs/manuals/linux/RHL-9-Manual/custom-guide/s1-raid-approaches.html

http://articles.techrepublic.com.com/5100-10878_11-5715216.html

Regards,
LIJEESH N G

[Bill Hassell]

RAID (Redundant Array of Inex[pensive Disks) is a broad term and comes in simple mirroring (RAID 1), striping (RAID 0), striped with interleaved or distributed parity (RAID 5) and others...See http://en.wikipedia.org/wiki/RAID

RAID 0 is a performance enhancer but no redundancy so all disks are available for storage. RAID 1 is simple mirroring so usable capacity is only half but if one fails, the system can continue using the other disk. RAID 0 and RAID 1 are standard features for LVM.

RAID 5 is very complex. It provides redundancy with a higher amount of usable storage. For example, 5 disks will provide 4 disk worth of storage. Any one disk can fail and the system will still see all the data. This is accomplished by computing and storing checksums that are stored across all the disks. This occurs when the data is first written. When data is read back, the checksums are verified as to the integrity of the data before sending the data to the host.

Additionally, software RAID 5 is severely compromised if any of the disks share the same disk interface. Compromised means that a single controller (with multiple disks) failure disables all the disks since RAID 5 cannot handle more than one disk failure at the time. And since a write must write to all the disks at the same time, a shared controller will be a severe bottleneck for performance.

So software RAID 5 is mostly a novelty. The cost to install extra controllers and the CPU load make it impractical.

Hardware RAID means that all of this special code plus separate buses for each drive are handled inside the RAID box.

[Kavita Poonia]

Thanks a lot for all the replies....now I am very much clear with the concept :)

Thanks & regards,
Kavita