Direct attached storage

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Direct Attached Storage ( DAS ) or Server Attached Storage refers to hard drives connected to a single computer that are located in a separate housing.

The usual interfaces are SCSI and increasingly SAS . However, all block-oriented transfer protocols for direct ( point-to-point ) connections are possible, for example ATA / ATAPI , FireWire , eSATA and USB with UAS . In contrast, there are Fiber Channel , iSCSI or FICON / ESCON , which are part of the network-oriented SAN concepts. The data carriers operated as DAS offer the same performance as drives built directly into the computer housing; In contrast, data carriers connected in the SAN usually have a slightly lower data transfer rate due to the additional effort involved in the network-oriented approach.

If the computer is a host , in contrast to Network Direct Attached Storage , Network Attached Storage or a Storage Area Network, other computers can usually only use the DAS hard disks via the network and via the host to which they are physically connected. An exception from the area of high availability are "Shared SCSI" installations in which two hosts have joint access to a SCSI bus and B. can also jointly access their drives using a suitable cluster file system (e.g. GFS ).

Another increasingly important use of direct attached storage ( DAS ) is the expansion of network attached storage ( NAS ) and storage area network ( SAN ) hard disk arrays. This is made possible by ever faster ( point-to-point ) connections such as SATA 3 and PCI Express . The NAS or SAN acts as a host and must accordingly offer host-compatible expansion interfaces (e.g. USB 3.0 or eSATA).

advantages

  • Low hardware expenditure.
  • Conceptually fast, the real speed depends on the technology used.
  • No additional protocol stack. The point-to-point connection is forwarded directly to the devices or the hard drives.

disadvantage

  • Only bound exclusively to one host

Disadvantages occur with DAS systems when several systems are used on different servers. All of these devices need to be monitored and managed with more effort.

  • Sometimes special protocol extensions are required (example: "SATA Port Multiplier")

Currently, many DAS systems based on eSATA use a “SATA port multiplier” internally, but the widely used Intel chipsets (ICHx) have mostly not implemented this protocol, so that other SATA controllers have to be retrofitted to provide this function to use.

See also

Individual evidence

  1. ↑ Issue 21/09 Hotline - SATA Port Multiplier . Heise Verlag, c't magazine. Retrieved December 24, 2013.