I've got a network 3.5 TB Lacie HDD which, in addition to a 10/10/1000
LAN port, also has 3 eSATA and 1 USB 2.0 ports. I am wondering if,
instead of going through a LAN and a router, I can simultaneously
connect sevaral PCs directly to the HDD through the eSATA/USB2.0, or
are the eSATAs/USB ports for daisy chaining? One of my PCs is old and
has a lame built-in slow network card, so, going through USB2.0 would
be faster.
eSATA is not on par with SAS nor USB, i.e. it can't allow your host PC
to see multiple drives on one eSATA cable -> plain daisy chaining is
certainly not possible, the Lacie box likely doesn't work as an "eSATA
hub".
I'd assume that the additional eSATA ports are indeed for additional
drives, useable by your RAID box - though it seems slightly odd, JBOD
expandability on such a small RAID box...
Regarding the possibility that the three eSATA ports are intended for
parallel access from multiple host computers: I can't rule that out
entirely. 3.5TB means that the external HDD is really a RAID box. I
haven't seen its user interface, so it's up to you to judge, if that
RAID box can create multiple "logical RAID volumes" and assign/map
those to individual eSATA channels. If the box also has USB and Gb
Eth, it might provide some intelligence in that vein - for multiple
access via eSATA / USB / GbEth (iSCSI? Samba?) from multiple hosts...
Another possibility with RAID boxes in general is, that indeed
multiple host channels can make a single common logical RAID volume
available to multiple host computers. That way, all three host
computers would see the same "SATA disk", would have access to the
same data space. I believe it's technically possible at this low level
(SATA / SCSI / block devices). But, making use of this for a
filesystem is a different matter. You need a "cluster filesystem" to
run on top of that - a filesystem that is *aware* of multiple
mounting, multiple host computers accessing the same data. This
requires some thorough block IO cache coherency protocol among the
host computers, with the locking likely running over Ethernet (= out
of band). Linux-based GFS and OCFS2 come to mind. Bare Windows NTFS
certainly won't cut it - that would result in data corruption, due to
cache incoherencies. There *are* software solutions like that for
Windows, one of them is the MetaSAN by Tiger Technology. Never tried
that though.
Frank Rysanek
