Hi, SSDs have low latency but a high price per GB, Traditional hard disks have high latency, but high sequential read/write speed and low price per GB. Is possibile to use a SSD for metadata, which requires many seeks and is relatively small, in a special "RAID mode" with a traditional hard disk for the extents of the real data? A cheap but performant SSD (maybe 32 GB) + a big and fast HD (maybe 1.5 TB, or two in RAID0 - 3TB ), wouldn''t create an array much cheaper than a ssd-only array of the same size, and much faster (in not-only-sequential workload) than one or two traditional HDs in RAID0? Would it work? Thank you for your precious time! Massimo Maggi massimo@mmmm.it -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Hello Massimo, Massimo Maggi wrote (ao):> SSDs have low latency but a high price per GB, > Traditional hard disks have high latency, but high sequential read/write > speed and low price per GB. > Is possibile to use a SSD for metadata, which requires many seeks and is > relatively small, in a special "RAID mode" with a traditional hard disk > for the extents of the real data? > A cheap but performant SSD (maybe 32 GB) + a big and fast HD (maybe 1.5 > TB, or two in RAID0 - 3TB ), wouldn''t create an array much cheaper than > a ssd-only array of the same size, and much faster (in > not-only-sequential workload) than one or two traditional HDs in RAID0? > Would it work?If you talk RAID0 (eg no redundancy), you could RAID0 one or several traditional disks, and use the SSD as a journal device. That would be ext3/4 only btw. With mdadm you could create a RAID1 and use --write-mostly: -W, --write-mostly subsequent devices listed in a --build, --create, or --add com- mand will be flagged as ''write-mostly''. This is valid for RAID1 only and means that the ''md'' driver will avoid reading from these devices if at all possible. This can be useful if mirror- ing over a slow link. Where the ''slow link'' would be the traditional disk. But this is raid1 and doesn''t help in your case (but couldn''t resist the need to mention it :-) Sander -- Humilis IT Services and Solutions http://www.humilis.net -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Massimo Maggi wrote:> Hi, > SSDs have low latency but a high price per GB, > Traditional hard disks have high latency, but high sequential read/write > speed and low price per GB. > Is possibile to use a SSD for metadata, which requires many seeks and is > relatively small, in a special "RAID mode" with a traditional hard disk > for the extents of the real data? > A cheap but performant SSD (maybe 32 GB) + a big and fast HD (maybe 1.5 > TB, or two in RAID0 - 3TB ), wouldn''t create an array much cheaper than > a ssd-only array of the same size, and much faster (in > not-only-sequential workload) than one or two traditional HDs in RAID0? > Would it work? > Thank you for your precious time! > Massimo Maggi > massimo@mmmm.itwell, it is not a "new" idea. yes people are thinking about it, but it is not the most critical work on the list of things to do for btrfs or any other linux fs. jim -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html