Re: [zfs-discuss] What is L2ARC write pattern?
From: zfs-discuss-boun...@opensolaris.org [mailto:zfs-discuss- boun...@opensolaris.org] On Behalf Of Jim Klimov One idea I have is that a laptop which only has a single HDD slot, often has SD/MMC cardreader slots. If populated with a card for L2ARC, can it be expected to boost the laptop's ZFS performance? You won't find that type of card with performance that's worth a damn. Worse yet, it will likely be extremely unreliable. In a SSD, all the performance and reliability come from intelligence in the controller, which emulates SATA HDD on one side, and manages Flash memory on the other side. Things like wear leveling, block mapping, garbage collection, etc, that's where all the performance comes from. You're not going to get it in a USB stick or a SD card. You're only going to get it in full size SSD's that consume power, and to some extent, the good stuff will cost more. (But of course, there's no way for the consumer to distinguish between paying for quality, and paying for marketing and margin, without trying it.) Even if you do try it, most likely you won't know the difference until a month later, having two identical systems with identical workload side-by-side. This is NOT to say the difference is insignificant; it's very significant, but without a point of reference, you don't have any comparison. All the published performance specs are fudged - but not lies - they represent optimal conditions, which are unrealistic. All the mfgrs are going to publish comparable specs, and none of them represent real life usage. ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
Re: [zfs-discuss] What is L2ARC write pattern?
2012-10-22 20:58, Brian wrote: hi jim, writes are sequential and to a ring buffer. reads of course would not be sequential, and would be intermixed with writes. Thanks... Do I get it correctly that if a block from L2ARC is requested by the readers, then it is fetched from the SSD and becomes a normal block in the ARC - thus the L2ARC entry won't be used again for this same block when it expires again from RAM ARC? Likewise, if the ring buffer is filled up and the block in L2ARC becomes overwritten, a possible read via the pointer from ARC to L2ARC fails (checksum mismatch), the pointer gets invalidated and pool disks get used to read the data from original source - right? If that all is the case, is there any sort of proactive expiration of values from L2ARC, or it is just not needed (useful data will travel from L2ARC to ARC and back again, and useless data will expire by being overwritten as the ring buffer completes its full cycle)? the digital photography cards probably aren't the fastest flash around, and may not be designed for a high number of (over)write cycles, but anything that can be configured as a vdev ought to work as l2arc. Well, I do photograph a lot and the consumer-grade cards begin to show deterioration in a couple of years of use. AFAIK the professional expensive ones also employ redundancy chips and thus improve speed and reliability - being more of an SSD in a non-SATA package ;) can't say how the system will behave if the flash is yanked during i/o. all the modules involved should handle that condition cleanly but it doesn't mean the use case will work. :) Thanks, //Jim ___ zfs-discuss mailing list zfs-discuss@opensolaris.org http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
