operation logs. Acts as a write buffer. Always on SSD
Content Cache (Extent Cache) for reading I/O. Primarily in Memory of CVM or SSD. Impact Size of CVM. Content cache is further divided into
- Single touch (in memory), smaller in size
- Multi touch (in memory + SSD), larger in size
Content cache does in-line de-duplication. Single touch keeps data which is read less frequently and eventually evicted. Multi touch keeps the data which is frequently accessed and in-frequently accessed data is moved to SSD. if data is accessed again, it is again moved to the top. However Data is never evicted out of SSD which helps in providing more cache hit ratio.
Capacity optimization engine responsible for compression
In-line and post process.
Combination of SSD and HDD
In-line, it is compressed before written. Post process: After data is written to disk (always happens in HDD tier)
Large write I/O (Large A) are first reviewed by COE and compressed in memory before written to Extent Store.
Small I/O (small A) represents random I/O. These are staged into Oplog and coalesce and review by COE, compressed and written to Extent Store.
Analogy of Small I/O: If you wish to load truck with boxes and If the boxes are large you can load truck in less than hour. If the boxes are small, it will take more than hour to load the truck and loads of efforts. You can reduces the efforts by putting all small boxes into large boxes.
Information lifecycle manager (ILM) will move the data to cold tier i.e. HDD tier. So once the data moves to cold tier, it is again reviewed by COE for compression.
If the compressed data needs to be read, “A” is uncompressed by COE and directly given to the requestor. In future if the data is accessed frequently, it is uncompressed and moved to Hot Tier then content Cache if frequency data access is increased.
Credits: http://stevenpoitras.com, nu.school