does iSCSI protocol itself or open-source iSCSI implementations like open-iscsi have a cache scheme?
since an iscsi initiator may access the same data many times, it does not have to get the data from remote target every time via network. so i am wondering if iSCSI provides a scheme by which clients can use a local disk to cache the hot data so that when the accessed data is stored in the local cache disk initiator can directly get the data from local disk, just like a web browser cache does. does anyone konw about this? thank you.
@Peter Breuer
hey Peter, thank you for your answer.
i knew that every block device can hold a page cache or buffer cache in memory ,and OS can use the memory cache to accelerate read/write operations, just as you said before.
iscsi device is also block device, but it's a special one, it accesses data via network after all. caching in memory is not enough due to its capacity limitation and volatility. so i think maybe using a local disk can cache more files or data from remote target.
i don't konw if iscsi itself supports a scheme like that, or if anybody has done things similar. so i am asking here.
@Peter
thank you for sharing so much content, and i will think about that.
@Peter
thanks, i think you have a very deep understanding of scsi layer and block layer, and that's what i am going to study, maybe i can learn a lot from you.
i take web browser as an example. as a client , the browser will cache some files that have been frequently requested from web server to client's local hard disk, so it doesn't need to get the same file from a remote web server via network, it just get the file locally to reduce access time. i don't know clearly if the local hard disk is used as a swap buffer and managed with main memory by OS, but i guess not. i think the local disk caching is managed by browser and acts as the browser's mechanism. the browser allocates a dedicated hard disk region to do caching and updates it.
so similarly, i have the thought, because both browser and iscsi initiator request data that don't store locally. the iscsi initiator(as a client) can also cache the frequently requested blocks from iscsi target(as a server ) to a local hard disk , just as a browser does. the main memory's capacity is limited, maybe it's not suitable for a big file.
so that's my hypothesis, if iscsi driver or scsi layer itself doesn't support that, maybe someone has researched this and added some mechanism to it.
just a hypothesis, and i haven't figured out the details. i think you are right to advise me to look into scsi driver, maybe the answer is there.
i hope someone has done things like this and shares with me here. i will appreciate that.
@Peter
i have read the paper before, this paper uses a auxiliary disk to do caching .
so i think i may also use a disk or ssd to help improve performance, and maybe i will improve the method in this paper.
Conference Paper A caching strategy to improve iSCSI performance
@Peter
sorry, please ignore the mark in this pdf file, that's not what i want you to look :)) that's not the key point.
the paper doesn't mention linux buffer system,it proposes itself's cache system.
"iCache uses a log disk along
with a piece of non-volatile RAM to cache the iSCSI traffic. Through efficient caching algorithm, iCache can significantly improve performance over current iSCSI systems"
as you see, it uses NVRAM and local disk to make a cache mechanism. that's what i want to study.
The Linux (or any other operating system) buffer cache will work just as well above iSCSI as above any other block device.
See Radkov et al., "A Performance Comparison of NFS and iSCSI for IP-Networked Storage" in FAST '04, https://www.usenix.org/legacy/events/fast04/tech/radkov.html
- Badges
- Science topic