Read the blog post: What You Need to Know Before Jumping into the Cloud Poolįile-tiering is a more advanced technology and is standards-based. You do not get the full benefit of eliminating cold data from storage, backups, migrations, and you get locked into the storage device. One way to resolve this would be to reserve the space left by all cold blocks tiered to the cloud, but this defeats the very purpose of tiering cold blocks.Īll of these limitations dramatically impact the actual savings you can receive from block-level tiering to just some storage efficiency. Once migrated, the rehydrated blocks will need to be tiered to the cloud from the new file server. Given that there will not likely be enough space on the original file server to re-hydrate all of the blocks moved over its lifetime, the only way to end-of-life the file server without losing data will be to iteratively rehydrate a small number of files at a time, migrate them to the new file server, and repeat. If the file server is to be end-of-life’d, all blocks tiered to the cloud from it must first be rehydrated, then migrated to the new file server. This defeats the purpose of tiering to an external storage, such as the cloud, since no space is saved on the original file server. Many operations like 3rd party backup software that operate at a file level require that the blocks of the file be brought back (re-hydrated) before they can be backed up. The moved blocks cannot be directly accessed from their new location, such as the cloud, because they are meaningless without all the other data blocks and the file context and attributes (the file’s metadata). All file access must be done through the original file server. Storage vendors are now using block-level tiering to move data out of the file server and into an object or cloud tier. Hot blocks and metadata are typically kept in the higher, faster, and more expensive storage tiers while cold blocks are migrated to lower, less expensive tiers. As the name implies, block-level tiering moves blocks between the various tiers to increase performance while reducing costs. Ext4 – Linux filesystem (when the configuration enables extents – the default in Linux since version 2.6.Block-level tiering was first introduced as a technique within a storage array to make the storage box more efficient by leveraging a mix of technologies such as more expensive SAS disks as well as cheaper SATA disks.EFS – Extent File System – SGI's first-generation file system for Irix.Btrfs – Extent-based copy-on-write (COW) file system for Linux.BFS – BeOS, Zeta and Haiku operating systems. ASM – Automatic Storage Management – Oracle's database-oriented filesystem.The systems supporting filesystem extents include the following: The system attempts to allocate the initial size as a contiguous area, although this may be split if contiguous space is not available. The initial allocation size and the size of additional extents to be allocated if required are specified by the user via Job Control Language. Files could originally have up to 16 extents, but this restriction has since been lifted. IBM OS/360 and successors allocate files in multiples of disk tracks or cylinders. CP/M's extents appear contiguously as a single block in the combined directory/allocation table, and they do not necessarily correspond to a contiguous data area on disk. As a similar design, the CP/M file system uses extents as well, but those do not correspond to the definition given above. Many modern fault-tolerant file systems also do copy-on-write, although that increases fragmentation. In order to resist fragmentation, several extent-based file systems do allocate-on-flush. But because the savings are small compared to the amount of stored data (for all file sizes in general) but make up a large portion of the metadata (for large files), the overall benefits in storage efficiency and performance are slight. Also, extent allocation results in less file fragmentation.Įxtent-based file systems can also eliminate most of the metadata overhead of large files that would traditionally be taken up by the block-allocation tree. The direct benefit is in storing each range compactly as two numbers, instead of canonically storing every block number in the range. A file can consist of zero or more extents one file fragment requires one extent. In computing, an extent is a contiguous area of storage reserved for a file in a file system, represented as a range of block numbers, or tracks on count key data devices. JSTOR ( December 2016) ( Learn how and when to remove this template message).Unsourced material may be challenged and removed.įind sources: "Extent" file systems – news Please help improve this article by adding citations to reliable sources. This article needs additional citations for verification.
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