Synchronous and Asynchronous writes

The effect of asynchronous writes on the performance of TSS device can be seen by comparing the RAID1, RAID5 and cRAID5 performance with synchronous and asynchronous writes (with max 8 async writes at a time). This results are summarized in Table 1.

The results indicate a significant improvement in performance of TSS, due to asynchronous writes in case of RAID1 and a moderate improvement in case of RAID5 and cRAID5. This improvement is achieved due to the reduction of waiting time through the use of asynchronous writes. In case of asynchronous writes, we have a separate thread running, which does the cleanup for each of the request. The time taken by this cleanup operation is not included in the results, as they happen asynchronously and they do not form a part of response time for the requests.

All of the following trace runs have been taken with async writes enabled in TSS.

Table 1: Effect of asynchronous writes on the performance of TSS (trace data from 6 [/var])

Type of	Avg. write time		Avg. I/O time
Device	per block (ms)		per block (ms)
	sync	async	sync	async
	writes	writes	writes	writes
RAID1	17.23	8.79	12.66	7.43
RAID5	21.98	19.32	14.87	13.47
cRAID5	94.87	74.36	65.90	54.00