A performance comparison of RAID-5 and Log-Structured-

Arrays

Jai Menon

Outline of the talk

• LSA Introduction• Analyze RAID-5 performance using an

analytical model.• Examine LSA performance.• Sensitivity of LSA performance to free

space and compression ratio achieved.• RAID5 with compression.• Results.

LSA

• Borrows heavily from Log-Structured-File systems.

• Combines LFS,RAID,Compression and Non-volatile cache.

• Updated data is written into new disk locations.

Raid-5 performance analysis

• RAID-5 controller built in from Non-Volatile-Storage(NVS).

• N+1 disks (N data+1 parity).• Fast write.• Dirty disk blocks.• Clean disk blocks.• Dirty blocks are written to disk using destaging.• NVS contains only data blocks.(not parity)

• Read hit:block requested is in cache.

• Read miss:block requested not in cache.

• Write hit: block written by a write request is in cache.

• Write miss: block written by a write request is not in cache.

• Write: data block is placed in the Non-Volatile cache, and destaged later.

• N+1 disks each is represented by 2 queues (read misses,destages).

• Read misses have higher priority.

• Non-preemptive priority.

• r. h., w. h., w. m. satisfied directly by cache.

• R. m. pass through the system then enter the read miss queue.

• A destage is 3 or 4 disk operations.

M/M/P system

P1

P2

Pn

LSA

• Data is stored in compressed form.• The system must keep a directory to locate

data items.• The host system writes records to the

subsystem, record is compressed and stored in controller cache.

• Logical track appears as a contiguous entity on disk.

LSA

• N+1 disks. Each is divided into segment columns.• A segment is the corresponding segment- columns

from n+1 disks.• One segment-column contains the parity of the

remaining segment-columns.• Parity segment columns are rotated among disks.• Logical devices are mapped and stored in the

LSA.

LSA

• The location of a logical track (LSA directory) changes over time.

• LSA directory is in NVS in disk controller.• When the fraction of cache occupied by

modified track exceeds a threshold, some number of modified tracks moved to a memory segment, from where they destaged to disk.

LSA

• Garbage collection.• If data can be written to any one of the disk

arrays, skew flattening can be better than RAID5(skew flattening within an array).

• Seek affinity is worse .• Disks see read misses,destage writes,reads

due to write misses and garbage collection reads and writes.

Effect of seek affinity

• Loss of seek affinity have a negative impact on performance.

• Performance of LSA without seek affinity is 50% higher at low I/O rates and 100% higher at high I/O rates than .5 seek affinity.

Effect of Free Space

• More free space implies lower average segment occupancy and hence lower garbage collection rate.

• If space allocated is 0.9 then garbage collection is responsible for 30% of the device utilization.

• If it is 0.7 then garbage collection takes 10%. The device can handle 25% more I/Os.

• Preferable 0.8 occupancy or less.

Effect of Compression Ratio

• The better the compression ratio the greater the free space and hence the better the performance.

• The better the compression ratio the better the cache hit ratio.

RAID5 with compression in cache

• A version of RAID5 in which data is stored compressed in cache.

• To write a record to disk, store it in compressed form but leave enough pad after it.

• We can always do update-in-place.• No disk space is saved.• No LSA directory is needed.• Improved performance or lower cost is possible.

Assumptions

• Storing of old data in RAID5 and LSA directory takes 17% of the cache.

• Systems with compressed cache have larger cache.

• RAID5 has flat skew within an array and some skew between arrays.LSA has no skew.

• Only LSA gets better transfer times.• RAID5 has better seek affinity.

results

• With no compression, LSA has a worse response time compared to RAID5, and about the same cost.

• With 3:1 compression LSA has better response time and a lower cost.However it has worse performance and better cost than RAID5 with compression.

Summary

• RAID5 with compression has better response time and throughput than RAID5.

• If data is already pre-compressed then LSA has worse response time and better throughput.

• If data is not pre-compressed then we can build an LSA 2X cheaper that has much worse response time but better throughput.

Comparison on a workload with flat skew

• With no data compression LSA and RAID5 have the same response time and almost the same throughput.

• With 3:1 compression, LSA has slightly worse response time and better throughput than RAID5 with compression.

• RAID5 with compression has better performance.

Questions

• What is LSA?

• Compare RAID5,LSA and RAID5 with compression with respect to response time and throughputs.

• What is skew , and how can it affect the performance of a disk array?