Choosing the Right Disk Technology in a High Availability Environment DRAFT Version 2.0, August 1996

Technical HPPA Newsletter # 218, June 17, 1995, "Update: Hot Swap
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DRAFT -- Revision 2.0
August 22, 1996Page 13
capacity disk drives, this results in a limit of 40 GB per F/W SCSI card. The number of
F/W SCSI cards in a system is constrained first by slots in the SPU and further by
performance considerations. Since this paper is concentrating on High Availability
solutions, the standalone disks must then be mirrored, cutting the maximum usable
capacity by 50%. This issue will be discussed further in the section on Capacity.
Mirrored JBODS and on-line replacement
The HASS has been designed to facilitate the on-line replacement of failed disk
mechanisms from a hardware perspective. Since the HASS does not have any built-in
data redundancy or protection, LVM mirroring must be used to provide that data
protection.
Older JBODs are not constructed for on-line replacement of failed disk mechanisms.
However, there is a procedure that allows the field service technician to approximate an
on-line disk replacement. Older disk trays have to be slid out the from rack without
disconnecting the SCSI bus and then the cover must be removed (which is necessary
before replacing a failed mechanism). The cover also has to be removed from external
disk towers to replace a failed disk.
Two supported procedures have been developed for the HASS and older disk trays and
towers that will allow either on-line or quiescent but powered-up replacement of failed
disk mechanisms. In another words, the computer system does NOT have to be
shutdown before replacing these mechanisms, thus saving time and interruption to the
users. The first procedure involves fewer steps, but the application must be quiesced
before the replacement. The second procedure allows the application to continue
running, but is more prone to user error due to the multiple complicated steps. These
procedures are documented in a Technical HPPA Newsletter article. PA NEWS is
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currently the only place that these procedures are documented.
Since JBODs are attractive to computer buyers for performance reasons, this new
procedure allows computer buyers to use standalone mirrored disks with almost the
same availability as with RAID disk arrays that support on-line mechanism replacement.
It is important to note, however, that there will be a scheduled interruption to the
application during the quiescent replacement of the failed disk mechanism with a JBOD
unless the more complex, but error-prone procedure is followed. Most organizations