IBM 000-8697 Manual de usuario descargar pdf (Pagina 75)

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Installation and Initial Configuration 1-41

OnLine Disk Space Allocation

Cooked ﬁles are unreliable because I/O on a cooked ﬁle is managed by the

UNIX operating system. A write to a cooked ﬁle can result in data being

writtento a memorybufferin the UNIXﬁlemanagerinstead of being written

immediately to disk. As a consequence, UNIX cannot guarantee that the

committed data has actually reached the disk. This is the problem. OnLine

recovery depends on the guarantee that data written to disk is actually on

disk. If, in the event of system failure, the data is not present on disk, the

OnLine automatic recovery mechanism could be unable to properly recover

the data. (The data in the UNIX buffer could be lost completely.) The end

result could be inconsistent data.

Performance degrades if you give up the efﬁciency beneﬁts of OnLine-

managed I/O. If you must use cooked UNIX ﬁles, try to store the least

frequently accessed data in the cooked ﬁles. Try to store the ﬁles in a ﬁle

system that is located near the center cylinders of the disk device, or in a ﬁle

system with minimal activity. In a learning environment, where reliability

and performancearenot criticalconcerns, cooked ﬁles are acceptable.(Since

OnLine manages the internal arrangement of data, you cannot edit the

contents of a cooked ﬁle.)

Signiﬁcant performance advantages and increased data reliability are

ensuredwhenOnLineperformsitsowndiskmanagementonrawdiskspace.

Raw disk space appears to your UNIX operating system as a disk device or

partofadiskdevice.In most operatingsystems,thedeviceisassociatedwith

both a block-special ﬁle and a character-special ﬁle in the /dev directory.

When you link your raw disk space to an OnLine chunk pathname, verify

thatyouuse the character-special ﬁle forthechunkname,nottheblock-special

ﬁle. (The character-specialﬁle candirectlytransferdata betweentheaddress

space of a user process and the disk using direct memory access (DMA),

which results in orders-of-magnitude better performance.)

How Much Disk Space Do You Need?

This section applies only if you are conﬁguring OnLine for a production

environment.Theﬁrststepin answering the question “How much space?”is

to calculate the size requirements of the root dbspace. The second step is to

estimate the total amount of disk space to allocate to all OnLine databases,

including space for overhead and growth.

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