Stephen John Smoogen wrote: > On 13 September 2017 at 09:25, hw <hw at gc-24.de> wrote: >> John R Pierce wrote: >>> >>> On 9/9/2017 9:47 AM, hw wrote: >>>> >>>> >>>> Isn´t it easier for SSDs to write small chunks of data at a time? >>>> The small chunk might fit into some free space more easily than >>>> a large one which needs to be spread out all over the place. >>> >>> >>> >>> the SSD collects data blocks being written and when a full flash block >>> worth of data is collected, often 256K to several MB, it writes them all at >>> once to a single contiguous block on the flash array, no matter what the >>> 'address' of the blocks being written is. think of it as a 'scatter-gather' >>> operation. >>> >>> different drive brands and models use different strategies for this, and >>> all this is completely opaque to the host OS so you really can't outguess or >>> manage this process at the OS or disk controller level. >>> >>> >> >> What if the collector is full? >> >> I understand that using small chunk sizes can reduce performance because >> many chunks need to be dealt with. Using large chunks would involve >> reading and writing larger amounts of data every time, and that also >> could reduce performance. >> >> With a chunk size of 1MB, disk access might amount to huge amounts of >> data being read and written unnecessarily. So what might be a good chunk >> size for SSDs? > > It will depend on the type of SSD. Ones with large cache and various > smarts (SAS Enterprise type) can take many different sizes. For SATA > ones it depends on what the cache and write of the SSD is and very few > of them seem to be the same. The SSD also has all kinds of logic which > moves data around constantly on disk to wipe level so it makes it > opaque. The people who have tested this usually have to burn through > an SSD set to get an idea about a particular 'run' of a model but it > doesn't go over every version of the model of SATA SSD. Hm, so much to SSDs ... I can only hope they will be replaced with something better. I have decided against putting anything onto these SSDs other than temporary data, but even for that, I would need to make an md-RAID, which I don´t want. It may work or not, and "may work" is not enough. If the performance on the hardware RAID isn´t as good, it can not get worse than it is now, and it may be even better than with the SSDs. I have two at home with the system installed on btrfs. I´m going to change that to md-RAID1 and xfs. Is there anything special involved in copying the system to another disk? Will 'cp -ax' do, or should I use rsync to copy xattrs etc.? Using the commonly used stripe size of 128kb is something I´d expect the SSDs being able to handle.