Thank you Stan. I agree with what you say. Sorry I was not more clear about my situation. Marcus is correct I'm trying to perform a data recovery operation from a failed Lacie box with two drives in a raid0 / chunk 512B configuration. I am totally aware of the complications involved but if I can somehow build a raid0 / chunk 512B array with mdadm using the raw Lacie dives then I can take it from there to recover the data. I have used another proprietary app and I am able to see the data using a custom chunk size of 1. So it should be doable I think. In looking at this more I tried to make a 512B stripe with dmsetup but it fails. It seems that "getconf PAGE_SIZE" which is set to 4096 is the lowest allowed stripe/chunk value. Maybe mdadm has the same size limitation. Thank you for your help. -V On 8/30/13, Marcus Sorensen <shadowsor@xxxxxxxxx> wrote: > Maybe he has raw drives that were in this NAS and is trying to match > the layout to recover something? I know that's probably not going to > work, certainly without a lot of other things going right, but its the > first thing that came to mind, given his reasoning and how he stated > it. > > On Aug 30, 2013 3:36 PM, "Stan Hoeppner" <stan@xxxxxxxxxxxxxxxxx> wrote: >> >> On 8/30/2013 2:32 PM, Veedar Hokstadt wrote: >> > Hello, >> > I would like to use mdadm to set up a raid0 with a 512B chunk size. >> > >> > I ask as my purpose is to mimic a raid0 config from a Lacie NAS box >> > that uses a 512B chunk size. >> >> Your reasoning is flawed. Why would you want to imitate a configuration >> that is inherently flawed? >> >> > The lowest chunk value mdadm will accpet is 4. Anything less and mdadm >> > gives an error "invalid chunk/rounding value" >> >> For good reason. >> >> > Is there any way to create a raid0 with a 512B chunk? >> >> First, if you're using RAID0 it absolutely must be assumed that you >> desire maximum speed, care nothing for redundancy, and you don't care if >> you lose your data when a disk fails because you have a full backup of >> the RAID0 filesystem. >> >> If you want speed, using RAID0 with a 512 byte chunk isn't going to >> achieve it. On the contrary, using such a small chunk will drop a >> hammer on your throughput because you're processing a much larger number >> of IOs per quantity of data transferred. This is extremely inefficient, >> and throughput drops. With RAID0 you typically want a very large chunk, >> the largest your drives can ingest efficiently in a single IO. I'll >> make an educated assumption that you plan to store media files on this >> array, probably DVDs/CDs, and/or use it as a DVR. In this case you want >> a large chunk, 512KB-1MB. >> >> However, you've stated you want to duplicate a NAS device. Consider >> that GbE using Rtl 81xx devices tops out at ~70-90 MBs application level >> throughput. Two modern drives in RAID0 with a proper chunk size can >> read/write at double that rate. Given this fact, why are you bothering >> with RAID0? You won't see any of the increased performance RAID0 can >> give you. In fact a single modern drive can saturate GbE. >> >> I assume you are using RAID0 simply as an inexpensive way to maximize >> your storage capacity. That's fine with backups or if you have the >> original media. If you don't, or don't want to go through the hassle of >> recreating your RAID0 after a disk failure and replacement, and copying >> all your files back to it, I suggest you use RAID1/5/6/10 instead. >> >> -- >> Stan >> >> >> >> -- >> To unsubscribe from this list: send the line "unsubscribe linux-raid" in >> the body of a message to majordomo@xxxxxxxxxxxxxxx >> More majordomo info at http://vger.kernel.org/majordomo-info.html > -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
