1 / 21

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

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.

moanna
Download Presentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. A performance comparison of RAID-5 and Log-Structured-Arrays Jai Menon

  2. 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.

  3. LSA • Borrows heavily from Log-Structured-File systems. • Combines LFS,RAID,Compression and Non-volatile cache. • Updated data is written into new disk locations.

  4. 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)

  5. 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.

  6. 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.

  7. 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.

  8. P1 P2 Pn M/M/P system

  9. 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.

  10. 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.

  11. 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.

  12. 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.

  13. 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. 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.

  21. 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?

More Related