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                                                                   Exhibit 10.31

                              ISILON SYSTEMS, INC.

                  FOURTH AMENDED AND RESTATED VOTING AGREEMENT

     This Fourth Amended and Restated Voting Agreement (the "Agreement") is made
as of the 19th day of July, 2006, by and among Isilon Systems, Inc., a Delaware
corporation (the "Company"), Paul Mikesell, Sujal Patel, Steve Goldman, Ellen M.
King, Jay M. King, Jon E. King, Jeanne K. King, Joan Eidem, LaVonne Molde,
William Goldman, Ruth Goldman and Eric Goldman (collectively, the "Common
Holders"), and the holders of shares of Series A, Series B, Series C, Series D
and Series E Preferred Stock of the Company listed on Exhibit A (collectively,
the "Investors" and individually, the "Investor").

                                    RECITALS

     A. Certain of the Investors (the "Prior Investors") hold shares of the
Company's Series A Preferred Stock (the "Series A Preferred Stock"), the
Company's Series B Preferred Stock (the "Series B Preferred Stock"), the
Company's Series C Preferred Stock (the "Series C Preferred Stock") or Company's
Series D Preferred Stock (the "Series D Preferred Stock") and possess certain
voting and other rights pursuant to a Third Amended and Restated Voting
Agreement dated as of May 10, 2005, among the Company, certain of the Common
Holders and the Prior Investors (the "Prior Agreement").

     B. The Common Holders executing this Agreement hold a majority of the
Common Stock of the Company (the "Common Stock") held by the Founders (as
defined in the Prior Agreement), the Prior Investors hold a majority of the
outstanding shares of the Series A Preferred Stock, Series B Preferred Stock,
Series C and Series D Preferred Stock, taken together and voting as a single
class on an as-converted basis, and each of them desires to amend and restate
the Prior Agreement and to accept the rights created pursuant hereto in lieu of
the rights granted to them under the Prior Agreement.

     C. The Investors purchasing Series E Preferred Stock of the Company (the
"Series E Investors") and the Company are parties to the Series E Preferred
Stock Purchase Agreement of even date herewith (the "Series E Agreement"), which
provides that as a condition to the closing of the sale of the Company's Series
E Preferred Stock (the "Series E Preferred Stock"), this Agreement must be
executed and delivered by the Series E Investors, Common Holders holding a
majority of the Common Stock held by the Common Holders (the "Common Holders'
Stock"), the Prior Investors holding a majority of the shares of the outstanding
Series A Preferred Stock, Series B Preferred Stock, Series C Preferred Stock and
Series D Preferred Stock taken together and voting as a single class on an
as-converted basis, and the Company.

     D. For the avoidance of doubt, for purposes of this agreement only, any
Investor whose Series A, Series B, Series C, Series D or Series E Preferred
Stock of the Company, as applicable, is converted by special mandatory
conversion to Series A-1, Series B-1, Series C-1, Series D-1 or Series E-1
Preferred Stock or any other Series of Preferred Stock, as applicable, issued
under Article IV, Section (B)8 or any similar successor provision of the
Company's Fifth Amended and

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Restated Certificate of Incorporation, shall be deemed hereunder to be a holder
of Series A, Series B, Series C, Series D or Series E Preferred Stock and shall
continue to be included in the definition of "Investor." All references to
Series A, Series B, Series C, Series D and Series E Preferred Stock hereunder
shall be deemed to include Series A-1, Series B-1, Series C-1, Series D-1 or
Series E-1 Preferred Stock issued upon conversion thereof and any other Series
of Preferred Stock issued under Article IV, Section (B)8 or any similar
successor provision of the Company's Fifth Amended and Restated Certificate of
Incorporation.

                                    AGREEMENT

     NOW, THEREFORE, in consideration of the mutual promises and covenants set
forth herein, the parties to the Prior Agreement hereby agree that the Prior
Agreement shall be superseded and replaced in its entirety by this Agreement,
and the parties hereto further agree as follows:

     1.   ELECTION OF DIRECTORS.

          1.1 BOARD REPRESENTATION. At each annual meeting of the stockholders
of the Company, or at any meeting of the stockholders of the Company at which
members of the Board of Directors of the Company are to be elected, or whenever
members of the Board of Directors are to be elected by written consent, the
Common Holders and the Investors agree to vote or act with respect to their
shares so as to elect:

               (a) one (1) member of the Company's Board of Directors designated
by Atlas Venture Fund V, L.P. or its affiliates ("Atlas") so long as Atlas owns
at least 3,000,000 shares of Series A Preferred Stock (as adjusted for stock
splits, stock dividends, reclassifications and the like), and such designee
shall initially be Barry Fidelman;

               (b) one (1) member of the Company's Board of Directors designated
by Madrona Venture Group or its affiliates ("Madrona") so long as Madrona owns
at least 3,000,000 shares of Series A Preferred Stock (as adjusted for stock
splits, stock dividends, reclassifications and the like), and such designee
shall initially be Matthew McIlwain;

               (c) one (1) member of the Company's Board of Directors designated
by Sequoia Capital or its affiliates ("Sequoia") so long as Sequoia owns at
least 3,000,000 shares of Series B Preferred Stock (as adjusted for stock
splits, stock dividends, reclassifications and the like), and such designee
shall initially be Greg McAdoo;

               (d) one (1) member of the Company's Board of Directors designated
by Lehman Brothers Venture Capital Group or its affiliates ("Lehman") so long as
Lehman owns at least 3,000,000 shares of Series C Preferred Stock (as adjusted
for stock splits, stock dividends, reclassifications and the like), and such
designee shall initially be Brian Paul;

               (e) one (1) member of the Company's Board of Directors designated
by the holders of a majority of the Common Stock of the Company, and such
designee shall initially be Sujal Patel;

               (f) one (1) member of the Company's Board of Directors to be an
individual holding the title of Chief Executive Officer of the Company, for so
long as such person

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remains Chief Executive Officer of the Company, and such designee shall
initially be Steven Goldman; and

               (g) one (1) independent member of the Company's Board of
Directors designated by the holders of a majority of the Series A Preferred
Stock, Series B Preferred Stock, Series C Preferred Stock, Series D Preferred
Stock, Series E Preferred Stock and the Common Stock, voting together as a
single class on an as-converted basis, and such designee shall initially be
William Ruckelshaus.

          1.2 APPOINTMENT OF DIRECTORS. In the event of the resignation, death,
removal or disqualification of a director selected by Atlas, Madrona, Sequoia,
Lehman, the holders of Common Stock or the holders of the Common Stock and
Preferred Stock (the "Designating Party"), such Designating Party shall promptly
nominate a new director and, after written notice to each of the parties hereto
of the new nominee to replace such director, each Investor and Common Holder
shall promptly vote its shares of capital stock of the Company to elect such
nominee to the Board of Directors.

          1.3 REMOVAL. The Designating Party may remove its respective
designated director or directors at any time and from time to time, with or
without cause (subject to the Bylaws of the Company as in effect from time to
time and any requirements of law), in such Designating Party's sole discretion,
and such Designating Party shall promptly nominate a new director and, after
written notice to each of the parties hereto of the new nominee to replace such
director, each Investor and Common Holder shall promptly vote its shares of
capital stock of the Company to elect such nominee to the Board of Directors.

     2.   ADDITIONAL REPRESENTATIONS AND COVENANTS.

          2.1 NO REVOCATION. The voting agreements contained herein are coupled
with an interest and may not be revoked during the term of this Agreement.

          2.2 AFFILIATES. For the purposes of this Agreement, "affiliate" means
a person that directly, or indirectly through one or more intermediaries,
controls, or is controlled by, or is under common control with, the person
specified and includes without limitation any person meeting the definition of
"affiliate" set forth in Rule 405 of the Securities Act of 1933, as amended.

          2.3 IRREVOCABLE PROXY. To secure the Common Holders' and the
Investors' obligations to vote their shares in accordance with this Agreement,
each Investor and Common Holder (each, a "Party") hereby appoints the Chairman
of the Board of Directors, or his or her designees, as such Party's true and
lawful proxy and attorney, with the power to act alone and with full power of
substitution, to vote all of such Party's shares in favor of the matters set
forth in this Agreement and to execute all appropriate instruments consistent
with this Agreement on behalf of such Party if, and only if, such Party fails to
vote all of such Party's shares or to execute such other instruments in
accordance with the provisions of this Agreement within five (5) days of the
Company's or any other party's written request for such Party's written consent
or signature. The proxy and power granted by each Common Holder and Investor
pursuant to this Section are coupled with an interest and are given to secure
the performance of such Parties' duties under this Agreement. Each such proxy
and power will be irrevocable for the term hereof.

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          2.4 CHANGE IN NUMBER OF DIRECTORS. The Common Holders and the
Investors will use their reasonable efforts to prevent, and will not vote for,
any amendment or change to the Company's Certificate of Incorporation or Bylaws
providing for the election of more or less than seven (7) directors, or any
other amendment or change to the Company's Certificate of Incorporation or
Bylaws inconsistent with the terms of this Agreement.

          2.5 LEGENDS. Each certificate representing shares of the Company's
capital stock held by the Common Holders or Investors or any assignee of the
Common Holders or Investors shall bear the following legend:

          "THE SHARES EVIDENCED HEREBY ARE SUBJECT TO A VOTING AGREEMENT BY AND
          AMONG THE COMPANY AND CERTAIN STOCKHOLDERS OF THE COMPANY (A COPY OF
          WHICH MAY BE OBTAINED FROM THE COMPANY), AND BY ACCEPTING ANY INTEREST
          IN SUCH SHARES THE PERSON ACCEPTING SUCH INTEREST SHALL BE DEEMED TO
          AGREE TO AND SHALL BECOME BOUND BY ALL THE PROVISIONS OF SAID VOTING
          AGREEMENT."

     3.   TERMINATION.

          3.1 TERMINATION EVENTS. This Agreement shall terminate upon the
earlier of:

               (a) A firm commitment underwritten public offering by the Company
of shares of its Common Stock pursuant to a registration statement on Form S-1
under the Securities Act of 1933, as amended, the per share public offering
price of which is not less than $3.6609 per share (appropriately adjusted for
any stock split, dividend, combination or other recapitalization) and which
results in aggregate cash proceeds to the Company of at least $25,000,000 (net
of underwriting discounts and commissions); or

               (b) A deemed liquidation pursuant to Article IV, Section
B(2)(c)(i) of the Company's Certificate of Incorporation, as such may be amended
from time to time.

          3.2 REMOVAL OF LEGEND. At any time after the termination of this
Agreement in accordance with Section 3.1, any holder of a stock certificate
legended pursuant to Section 2.5 may surrender such certificate to the Company
for removal of the legend, and the Company will duly reissue a new certificate
without the legend.

     4.   MISCELLANEOUS.

          4.1 SUCCESSORS AND ASSIGNS. The terms and conditions of this Agreement
shall inure to the benefit of and be binding upon the respective successors and
assigns of the parties. Nothing in this Agreement, express or implied, is
intended to confer upon any party other than the parties hereto or their
respective successors and assigns any rights, remedies, obligations, or
liabilities under or by reason of this Agreement, except as expressly provided
in this Agreement.

          4.2 AMENDMENTS AND WAIVERS. Any term hereof may be amended or waived
only with the written consent of each of (a) the Company, (b) the holders of a
majority of the shares of Common Holders' Stock, and (c) the holders of a
majority of the then outstanding shares of

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Series A Preferred Stock, Series B Preferred Stock, Series C Preferred Stock,
Series D Preferred and Series E Preferred Stock, taken together and voting as a
single class on an as-converted basis; provided, however, that notwithstanding
the foregoing, Section 1.1(a) of this Agreement shall not be amended and the
observance of any term thereof shall not be waived without the written consent
of Atlas; Section 1.1(b) of this Agreement shall not be amended and the
observance of any term thereof shall not be waived without the written consent
of Madrona; Section 1.1(c) of this Agreement shall not be amended and the
observance of any term thereof shall not be waived without the written consent
of Sequoia; and Section 1.1(d) of this Agreement shall not be amended and the
observance of any term thereof shall not be waived without the written consent
of Lehman. Any amendment or waiver effected in accordance with this Section 4.2
shall be binding upon the Company, the holders of Series A Preferred Stock,
Series B Preferred Stock, Series C Preferred Stock, Series D Preferred Stock and
Series E Preferred Stock, and any holder of Common Holders' Stock, and each of
their respective successors and assigns.

          4.3 NOTICES. Any notice required or permitted by this Agreement shall
be in writing and shall be deemed sufficient on the date of delivery, when
delivered personally or by overnight courier or sent by telegram or fax (upon
customary confirmation of receipt), or forty-eight (48) hours after being
deposited in the U.S. mail, as certified or registered mail, with postage
prepaid, and addressed to the party to be notified at such party's address or
fax number as set forth on the signature page hereto, or as subsequently
modified by written notice.

          4.4 SEVERABILITY. If one or more provisions of this Agreement are held
to be unenforceable under applicable law, the parties agree to renegotiate such
provision in good faith. In the event that the parties cannot reach a mutually
agreeable and enforceable replacement for such provision, then (a) such
provision shall be excluded from this Agreement, (b) the balance of the
Agreement shall be interpreted as if such provision were so excluded and (c) the
balance of the Agreement shall be enforceable in accordance with its terms.

          4.5 ATTORNEYS' FEES. If any action at law or in equity is necessary to
enforce or interpret the terms of this Agreement, the prevailing party shall be
entitled to reasonable attorneys' fees, costs and necessary disbursements in
addition to any other relief to which such party may be entitled.

          4.6 SPECIFIC ENFORCEMENT. It is agreed that monetary damages would not
adequately compensate an injured party for the breach of this Agreement by any
party, that this Agreement shall be specifically enforceable, and that any
breach or threatened breach of this Agreement shall be the proper subject of a
temporary or permanent injunction or restraining order. Furthermore, each party
to this Agreement hereby waives any claim or defense that there is an adequate
remedy at law for any breach or threatened breach of this Agreement.

          4.7 GOVERNING LAW. This Agreement and all acts and transactions
pursuant hereto and the rights and obligations of the parties hereto shall be
governed, construed and interpreted in accordance with the laws of the State of
Delaware, without giving effect to principles of conflicts of law.

          4.8 COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall be deemed an original and all of which
together shall constitute one instrument.

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          4.9 TITLES AND SUBTITLES. The titles and subtitles used in this
Agreement are used for convenience only and are not to be considered in
construing or interpreting this Agreement.

          4.10 DELAYS OR OMISSIONS. No delay or omission to exercise any right,
power or remedy accruing to any party under this Agreement, upon any breach or
default of any other party under this Agreement, shall impair any such right,
power or remedy of such non-breaching or non-defaulting party nor shall it be
construed to be a waiver of any such breach or default, or an acquiescence
therein, or of or in any similar breach or default thereafter occurring; nor
shall any waiver of any single breach or default be deemed a waiver of any other
breach or default theretofore or thereafter occurring. Any waiver, permit,
consent or approval of any kind or character on the part of any party of any
breach or default under this Agreement, or any waiver on the part of any party
of any provisions or conditions of this Agreement, must be in writing and shall
be effective only to the extent specifically set forth in such writing. All
remedies, either under this Agreement or by law or otherwise afforded to any
party, shall be cumulative and not alternative.

                            [Signature Page Follows]

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     The parties hereto have executed this Fourth Amended and Restated Voting
Agreement as of the date first written above.

                                        COMPANY:

                                        ISILON SYSTEMS, INC.,
                                        a Delaware corporation


                                        By: /s/ Steven Goldman
                                            ------------------------------------
                                            Steven Goldman, President


                                        COMMON HOLDERS:


                                        /s/ Sujal Patel
                                        ----------------------------------------
                                        Sujal Patel


                                        /s/ Steve Goldman
                                        ----------------------------------------
                                        Steve Goldman


                                        INVESTORS:

                                        GRANITE GLOBAL VENTURES II L.P.

                                        By: Granite Global Ventures II L.L.C.,
                                        its General Partner


                                        By: /s/ Glenn Solomon
                                            ------------------------------------
                                            Glenn Solomon
                                            Managing Director


                                        GGV II ENTREPRENEURS FUND L.P.

                                        By: Granite Global Ventures II L.L.C.,
                                        its General Partner


                                        By: /s/ Glenn Solomon
                                            ------------------------------------
                                            Glenn Solomon
                                            Managing Director

   [SIGNATURE PAGE TO ISILON SYSTEMS, INC. FOURTH AMENDED AND RESTATED VOTING
                                   AGREEMENT]

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                                        INVESTORS:

                                        FOCUS VENTURES II, L.P.

                                        By: Focus Ventures Partners II, L.P.


                                        By: /s/ James H. Boettcher
                                            ------------------------------------
                                            James H. Boettcher
                                            General Partner


                                        FV INVESTORS II QP, L.P.

                                        By: Focus Ventures Partners II, L.P.


                                        By: /s/ James H. Boettcher
                                            ------------------------------------
                                            James H. Boettcher
                                            General Partner


                                        FV INVESTORS II A, L.P.

                                        By: Focus Ventures Partners II, L.P.


                                        By: /s/ James H. Boettcher
                                            ------------------------------------
                                            James H. Boettcher
                                            General Partner


                                        INVESTORS:

                                        LEHMAN BROTHERS VENTURE PARTNERS 2003-C,
                                        L.P.

                                        By: Lehman Brothers Venture GP
                                            Partnership 2003 L.P.,
                                        Its: General Partner

                                        By: Lehman Brothers Venture Associates
                                            2003 LLC,
                                        Its: General Partner


                                        By: /s/ Brian Paul
                                            ------------------------------------
                                            Brian Paul
                                            Vice President

   [SIGNATURE PAGE TO ISILON SYSTEMS, INC. FOURTH AMENDED AND RESTATED VOTING
                                   AGREEMENT]

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                                        LEHMAN BROTHERS VENTURE PARTNERS 2003-P,
                                        L.P.

                                        By: Lehman Brothers Venture GP
                                            Partnership 2003 L.P.,
                                        Its: General Partner

                                        By: Lehman Brothers Venture Associates
                                            2003 LLC,
                                        Its: General Partner


                                        By: /s/ Brian Paul
                                            ------------------------------------
                                            Brian Paul
                                            Vice President


                                        LEHMAN BROTHERS VENTURE CAPITAL 2003
                                        PARTNERSHIP

                                        By: Lehman Brothers Venture Associates
                                            2003 LLC,
                                        Its: General Partner

                                        By: LB I Group Inc.,
                                        Its: General Partner


                                        By: /s/ Brian Paul
                                            ------------------------------------
                                            Brian Paul
                                            Vice President


                                        LEHMAN BROTHERS P.A. LLC

                                        By: /s/ Brian Paul
                                            ------------------------------------
                                            Brian Paul
                                            Vice President

   [SIGNATURE PAGE TO ISILON SYSTEMS, INC. FOURTH AMENDED AND RESTATED VOTING
                                   AGREEMENT]

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                                        LEHMAN BROTHERS VENTURE CAPITAL
                                        PARTNERS II, L.P.

                                        By: Lehman Brothers Venture Associates
                                            II LLC,
                                        Its: General Partner


                                        By: /s/ Brian Paul
                                            ------------------------------------
                                            Brian Paul
                                            Vice President


                                        LEHMAN BROTHERS PARTNERSHIP ACCOUNT
                                        2000/2001, L.P.

                                        By: Lehman Brothers Partnership GP
                                            2000/2001, L.P.,
                                        Its: General Partner

                                        By: LB I Group Inc.,
                                        Its: General Partner


                                        By: /s/ Brian Paul
                                            ------------------------------------
                                            Brian Paul
                                            Vice President


                                        LEHMAN BROTHERS OFFSHORE PARTNERSHIP
                                        ACCOUNT 2000/2001, L.P.

                                        By: Lehman Brothers Offshore
                                            Partnership GP 2000/2001, L.P.,

                                        Its: General Partner

                                        By: Lehman Brothers Offshore Partners
                                            Ltd.,
                                        Its: General Partner


                                        By: /s/ Brian Paul
                                            ------------------------------------
                                            Brian Paul
                                            Vice President

   [SIGNATURE PAGE TO ISILON SYSTEMS, INC. FOURTH AMENDED AND RESTATED VOTING
                                   AGREEMENT]

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                                        INVESTORS:

                                        SEQUOIA CAPITAL X
                                        SEQUOIA TECHNOLOGY PARTNERS X
                                        SEQUOIA CAPITAL X PRINCIPALS FUND

                                        By: SCX Management, L.L.C.
                                            A Delaware Limited Liability Company
                                            General Partner of Each


                                        By: /s/ Doug Leone
                                            ------------------------------------
                                            Doug Leone
                                        Title: Managing Member


                                        INVESTORS:

                                        ATLAS VENTURE FUND V, L.P.
                                        ATLAS VENTURE PARALLEL FUND V-A C.V.
                                        ATLAS VENTURE ENTREPRENEURS' FUND V,
                                        L.P.

                                        By: Atlas Venture Associates V, L.P.
                                        Its: General Partner

                                        By: Atlas Venture Associates V, Inc.
                                        Its: General Partner


                                        By: /s/ Jeanne Larkin Henry
                                            ------------------------------------
                                            Jeanne Larkin Henry
                                        Title: VP


                                        INVESTORS:

                                        MADRONA VENTURE FUND I-A, LP

                                        By: Madrona Investment Partners, LLC,
                                        Its: General Partner


                                        By: /s/ Matthew S. McIlwain
                                            ------------------------------------
                                            Matthew S. McIlwain
                                        Title: Managing Director

   [SIGNATURE PAGE TO ISILON SYSTEMS, INC. FOURTH AMENDED AND RESTATED VOTING
                                   AGREEMENT]

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                                        MADRONA VENTURE FUND I-B, LP

                                        By: Madrona Investment Partners, LLC,
                                        Its: General Partner


                                        By: /s/ Matthew S. McIlwain
                                            ------------------------------------
                                            Matthew S. McIlwain
                                        Title: Managing Director


                                        MADRONA MANAGING DIRECTOR FUND, LLC


                                        By: /s/ Matthew S. McIlwain
                                            ------------------------------------
                                            Matthew S. McIlwain
                                        Title: Managing Director


                                        INVESTORS:

                                        ROLLING BAY VENTURES LLC
                                        a Washington limited liability company


                                        By: /s/ Geoffrey R. Entress
                                            ------------------------------------
                                            Geoffrey R. Entress
                                        Title: Managing Member


                                        /s/ Sujal Patel
                                        ----------------------------------------
                                        Sujal Patel


                                        WS INVESTMENT COMPANY, LLC


                                        By: /s/ Craig Sherman
                                            ------------------------------------
                                            Craig Sherman
                                            Member

   [SIGNATURE PAGE TO ISILON SYSTEMS, INC. FOURTH AMENDED AND RESTATED VOTING
                                   AGREEMENT]