Hi, > The major reason is, it needs to read full 512KB segment to calculate checksum to > know if the log isn't half written. > (Read 500GB SSD that performs 500MB/sec seqread spends 1000secs) I've just measured how long the cache resuming is. I use 2GB SSD for the cache device. 512KB seqread over the cache device: 8.252sec (242MB/sec) Resume when all caches are dirty: 10.339sec Typically, if you use 128GB SSD it will be 5-10 minutes. As I predicted, resuming time is close to seqread. In other words, it's fully IO-bound. (If you read the code you will notice that it first searchs for the oldest log as the starting point. It's 4KB metadata reads but spends to some extent. The other 2 sec is thought to be spent by this) - Akira On 12/13/14 11:07 PM, Akira Hayakawa wrote: > Hi, > > Jianjian, You really get a point at the fundamental design. > >> If I understand it correctly, the whole idea indeed is very simple, >> the consumer/provider and circular buffer model. use SSD as a circular >> write buffer, write flush thread stores incoming writes to this buffer >> sequentially as provider, and writeback thread write those data logs >> sequentially into backing device as consumer. >> >> If writeboost can do that without any random writes, then probably it >> can save SSD/FTL of doing a lot of dirty jobs, and utilize the faster >> sequential read/write performance from SSD. That'll be awesome. >> However, I saw every data log segment in its design has meta data >> header, like dirty_bits, so I guess writeboost has to randomly write >> those data headers of stored data logs in SSD; also, splitting all bio >> into 4KB will hurt its ability to get max raw SSD throughput, modern >> NAND Flash has pages much bigger than 4KB; so overall I think the >> actual benefits writeboost gets from this design will be discounted. > You understand *almost* correctly. > > Writeboost has two circular buffers, not one; RAM buffers and SSD. > The incoming bio is split into 4KB chunks at the virtual make_request > and are NOT directly remapped to the SSD. > As you mentioned, if I designed so, many update on the metadata happens. > That's really bad since SSD is very bad at small update. > > Actually, the 4KB bio is first stored in RAM buffer, which is 512KB large. > There are (512-4)/4=127 4KB bio data stored in the RAM buffer and 4KB metadata > section at the head is made after that. > > The RAM buffer is now called "log" and as you mentioned, flushed to the SSD > as 512KB sequential write. This definitely maximizes throughput and lifetime. > > Unfortunately, this is not always the case because of barrier request handlings. > But, when the writes is really heavy (e.g. massive dirty page writeback), > Writeboost works as above. > >> The good thing is that it seems writeboost doesn't use garbage >> collection to clean old invalid logs, this will avoid the double >> garage collection effect other caching module has, which essentially >> both caching module and internal SSD will perform garbage collections >> twice. > Yes. And I believe SSDs can remove wear-leveling if I used it as fairly sequential. > Am I right? Indeed, Writeboost is really SSD frinedly. > >> And one question, how long will be data logs replay time during init, >> if SSD is almost full of dirty data logs? > Sorry, I don't have a data now but it's slow as you may imagine. > I will measure the time on later. > > The major reason is, it needs to read full 512KB segment to calculate checksum to > know if the log isn't half written. > (Read 500GB SSD that performs 500MB/sec seqread spends 1000secs) > I think making the procedure done in parallel to exploit the full internal parallelism > inside SSD may improve performance but it's just the matter of coefficient down from 1 to 1/n. > Definitely, Writeboost isn't fit for a machine that needs reboot frequently (e.g. desktop). > > There is a way to reduce the init time. We can dump "what is the latest log written back" > on the superblock. This can skip readings that aren't essential. > > The corresponding code is replay_log_on_cache() function. Please read if you are > interested. > > Thanks, > > - Akira > > On 12/13/14 3:45 PM, Jianjian Huo wrote: >> If I understand it correctly, the whole idea indeed is very simple, >> the consumer/provider and circular buffer model. use SSD as a circular >> write buffer, write flush thread stores incoming writes to this buffer >> sequentially as provider, and writeback thread write those data logs >> sequentially into backing device as consumer. >> >> If writeboost can do that without any random writes, then probably it >> can save SSD/FTL of doing a lot of dirty jobs, and utilize the faster >> sequential read/write performance from SSD. That'll be awesome. >> However, I saw every data log segment in its design has meta data >> header, like dirty_bits, so I guess writeboost has to randomly write >> those data headers of stored data logs in SSD; also, splitting all bio >> into 4KB will hurt its ability to get max raw SSD throughput, modern >> NAND Flash has pages much bigger than 4KB; so overall I think the >> actual benefits writeboost gets from this design will be discounted. >> >> The good thing is that it seems writeboost doesn't use garbage >> collection to clean old invalid logs, this will avoid the double >> garage collection effect other caching module has, which essentially >> both caching module and internal SSD will perform garbage collections >> twice. >> >> And one question, how long will be data logs replay time during init, >> if SSD is almost full of dirty data logs? >> >> Jianjian >> >> On Fri, Dec 12, 2014 at 7:09 AM, Akira Hayakawa <ruby.wktk@xxxxxxxxx> wrote: >>>> However, after looking at the current code, and using it I think it's >>>> a long, long way from being ready for production. As we've already >>>> discussed there are some very naive design decisions in there, such as >>>> copying every bio payload to another memory buffer, splitting all io >>>> down to 4k. Think about the cpu overhead and memory consumption! >>>> Think about how it will perform when memory is constrained and it >>>> can't allocate many of those rambufs! I'm sure more issues will be >>>> found if I read further. >>> These decisions are made based on measurement. They are not naive. >>> I am a man who dislikes performance optimization without measurement. >>> As a result, I regard things brought by the simplicity much important >>> than what's from other design decisions possible. >>> >>> About the CPU consumption, >>> the average CPU consumption while performing random write fio >>> with consumer level SSD is only 3% or so, >>> which is 5 times efficient than bcache per iops. >>> >>> With RAM-backed cache device, it reaches about 1.5GB/sec throughput. >>> Even in this case the CPU consumption is only 12%. >>> Please see this post, >>> http://www.redhat.com/archives/dm-devel/2014-February/msg00000.html >>> >>> I don't think the CPU consumption is small enough to ignore. >>> >>> About the memory consumption, >>> you seem to misunderstand the fact. >>> The rambufs are not dynamically allocated but statically. >>> The default amount is 8MB and this is usually not to argue. >>> >>>> Mike raised the question of why you want this in the kernel so much? >>>> You'd find none of the distros would support it; so it doesn't widen >>>> your audience much. It's far better for you to maintain it outside of >>>> the kernel at this point. Any users will be bold, adventurous people, >>>> who will be quite capable of building a kernel module. >>> Some people deploy Writeboost in their daily use. >>> The sound of "log-structured" seems to easily attract storage guys' attention. >>> If this driver is merged into upstream, I think it gains many audience and >>> thus feedback. >>> When my driver was introduced by Phoronix before, it actually drew attentions. >>> They must wait for Writeboost become available in upstream. >>> http://www.phoronix.com/scan.php?page=news_item&px=MTQ1Mjg >>> >>>> I'm sorry to have disappointed you so, but if I let this go upstream >>>> it would mean a massive amount of support work for me, not to mention >>>> a damaged reputation for dm. >>> If you read the code further, you will find how simple the mechanism is. >>> Not to mention the code itself is. >>> >>> - Akira >>> >>> On 12/12/14 11:24 PM, Joe Thornber wrote: >>>> On Fri, Dec 12, 2014 at 09:42:15AM +0900, Akira Hayakawa wrote: >>>>> The SSD-caching should be log-structured. >>>> >>>> No argument there, and this is why I've supported you with >>>> dm-writeboost over the last couple of years. >>>> >>>> However, after looking at the current code, and using it I think it's >>>> a long, long way from being ready for production. As we've already >>>> discussed there are some very naive design decisions in there, such as >>>> copying every bio payload to another memory buffer, splitting all io >>>> down to 4k. Think about the cpu overhead and memory consumption! >>>> Think about how it will perform when memory is constrained and it >>>> can't allocate many of those rambufs! I'm sure more issues will be >>>> found if I read further. >>>> >>>> I'm sorry to have disappointed you so, but if I let this go upstream >>>> it would mean a massive amount of support work for me, not to mention >>>> a damaged reputation for dm. >>>> >>>> Mike raised the question of why you want this in the kernel so much? >>>> You'd find none of the distros would support it; so it doesn't widen >>>> your audience much. It's far better for you to maintain it outside of >>>> the kernel at this point. Any users will be bold, adventurous people, >>>> who will be quite capable of building a kernel module. >>>> >>>> - Joe >>>> >>> >>> -- >>> dm-devel mailing list >>> dm-devel@xxxxxxxxxx >>> https://www.redhat.com/mailman/listinfo/dm-devel >> >> -- >> dm-devel mailing list >> dm-devel@xxxxxxxxxx >> https://www.redhat.com/mailman/listinfo/dm-devel >> > -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
